erl_syntax

Abstract Erlang syntax trees.

This module defines an abstract data type for representing Erlang source code as syntax trees, in a way that is backwards compatible with the data structures created by the Erlang standard library parser module erl_parse (often referred to as "parse trees", which is a bit of a misnomer). This means that all erl_parse trees are valid abstract syntax trees, but the reverse is not true: abstract syntax trees can in general not be used as input to functions expecting an erl_parse tree. However, as long as an abstract syntax tree represents a correct Erlang program, the function revert/1 should be able to transform it to the corresponding erl_parse representation.

A recommended starting point for the first-time user is the documentation of the syntaxTree() data type, and the function type/1.

NOTES:

This module deals with the composition and decomposition of syntactic entities (as opposed to semantic ones); its purpose is to hide all direct references to the data structures used to represent these entities. With few exceptions, the functions in this module perform no semantic interpretation of their inputs, and in general, the user is assumed to pass type-correct arguments - if this is not done, the effects are not defined.

With the exception of the erl_parse data structures, the internal representations of abstract syntax trees are subject to change without notice, and should not be documented outside this module. Furthermore, we do not give any guarantees on how an abstract syntax tree may or may not be represented, with the following exceptions: no syntax tree is represented by a single atom, such as none, by a list constructor [X | Y], or by the empty list []. This can be relied on when writing functions that operate on syntax trees.

erl_parse() = parse_tree() (see module erl_parse)

The "parse tree" representation built by the Erlang standard library parser erl_parse. This is a subset of the syntaxTree type.

syntaxTree()

An abstract syntax tree. The erl_parse "parse tree" representation is a subset of the syntaxTree() representation.

Every abstract syntax tree node has a type, given by the function type/1. Each node also has associated attributes; see get_attrs/1 for details. The functions make_tree/2 and subtrees/1 are generic constructor/decomposition functions for abstract syntax trees. The functions abstract/1 and concrete/1 convert between constant Erlang terms and their syntactic representations. The set of syntax tree nodes is extensible through the tree/2 function.

A syntax tree can be transformed to the erl_parse representation with the revert/1 function.

syntaxTreeAttributes()

This is an abstract representation of syntax tree node attributes; see the function get_attrs/1.

Returns the syntax tree corresponding to an Erlang term. Term must be a literal term, i.e., one that can be represented as a source code literal. Thus, it may not contain a process identifier, port, reference, binary or function value as a subterm. The function recognises printable strings, in order to get a compact and readable representation. Evaluation fails with reason badarg if Term is not a literal term.

See also: concrete/1, is_literal/1.

Appends the term Annotation to the list of user annotations of Node.

Note: this is equivalent to set_ann(Node, [Annotation | get_ann(Node)]), but potentially more efficient.

See also: get_ann/1, set_ann/2.

Appends Comments to the post-comments of Node.

Note: This is equivalent to set_postcomments(Node, get_postcomments(Node) ++ Comments), but potentially more efficient.

See also: add_precomments/2, comment/2, get_postcomments/1, join_comments/2, set_postcomments/2.

Appends Comments to the pre-comments of Node.

Note: This is equivalent to set_precomments(Node, get_precomments(Node) ++ Comments), but potentially more efficient.

See also: add_postcomments/2, comment/2, get_precomments/1, join_comments/2, set_precomments/2.

Creates an abstract function application expression. If Arguments is [A1, ..., An], the result represents "Operator(A1, ..., An)".

See also: application/3, application_arguments/1, application_operator/1.

Types:

Creates an abstract function application expression. If Module is none, this is call is equivalent to application(Function, Arguments), otherwise it is equivalent to application(module_qualifier(Module, Function), Arguments).

(This is a utility function.)

See also: application/2, module_qualifier/2.

Returns the list of argument subtrees of an application node.

See also: application/2.

Returns the operator subtree of an application node.

Note: if Node represents "M:F(...)", then the result is the subtree representing "M:F".

See also: application/2, module_qualifier/2.

Creates an abstract arity qualifier. The result represents "Body/Arity".

See also: arity_qualifier_argument/1, arity_qualifier_body/1.

Returns the argument (the arity) subtree of an arity_qualifier node.

See also: arity_qualifier/2.

Returns the body subtree of an arity_qualifier node.

See also: arity_qualifier/2.

Types:

Creates an abstract atom literal. The print name of the atom is the character sequence represented by Name.

See also: atom_literal/1, atom_name/1, atom_value/1, is_atom/2.

Returns the literal string represented by an atom node. This includes surrounding single-quote characters if necessary.

Note that e.g. the result of atom("x\ny") represents any and all of 'x\ny', 'x\12y', 'x\012y' and 'x\^Jy\'; cf. string/1.

See also: atom/1, string/1.

Returns the printname of an atom node.

See also: atom/1.

Returns the value represented by an atom node.

See also: atom/1.

Equivalent to attribute(Name, none).

Types:

Creates an abstract program attribute. If Arguments is [A1, ..., An], the result represents "-Name(A1, ..., An).". Otherwise, if Arguments is none, the result represents "-Name.". The latter form makes it possible to represent preprocessor directives such as "-endif.". Attributes are source code forms.

Note: The preprocessor macro definition directive "-define(Name, Body)." has relatively few requirements on the syntactical form of Body (viewed as a sequence of tokens). The text node type can be used for a Body that is not a normal Erlang construct.

See also: attribute/1, attribute_arguments/1, attribute_name/1, is_form/1, text/1.

Returns the list of argument subtrees of an attribute node, if any. If Node represents "-Name.", the result is none. Otherwise, if Node represents "-Name(E1, ..., En).", [E1, ..., E1] is returned.

See also: attribute/1.

Returns the name subtree of an attribute node.

See also: attribute/1.

Creates an abstract binary-object template. If Fields is [F1, ..., Fn], the result represents "<<F1, ..., Fn>>".

See also: binary_field/2, binary_fields/1.

Creates an abstract binary comprehension. If Body is [E1, ..., En], the result represents "<<Template || E1, ..., En>>".

See also: binary_comp_body/1, binary_comp_template/1, generator/2.

Returns the list of body subtrees of a binary_comp node.

See also: binary_comp/2.

Returns the template subtree of a binary_comp node.

See also: binary_comp/2.

Equivalent to binary_field(Body, []).

Creates an abstract binary template field. If Types is the empty list, the result simply represents "Body", otherwise, if Types is [T1, ..., Tn], the result represents "Body/T1-...-Tn".

See also: binary/1, binary_field/1, binary_field/3, binary_field_body/1, binary_field_size/1, binary_field_types/1.

Types:

Creates an abstract binary template field. If Size is none, this is equivalent to "binary_field(Body, Types)", otherwise it is equivalent to "binary_field(size_qualifier(Body, Size), Types)".

(This is a utility function.)

See also: binary/1, binary_field/2, size_qualifier/2.

Returns the body subtree of a binary_field.

See also: binary_field/2.

Returns the size specifier subtree of a binary_field node, if any. If Node represents "Body:Size" or "Body:Size/T1, ..., Tn", the result is Size, otherwise none is returned.

(This is a utility function.)

See also: binary_field/2, binary_field/3.

Returns the list of type-specifier subtrees of a binary_field node. If Node represents ".../T1, ..., Tn", the result is [T1, ..., Tn], otherwise the result is the empty list.

See also: binary_field/2.

Returns the list of field subtrees of a binary node.

See also: binary/1, binary_field/2.

Creates an abstract binary_generator. The result represents "Pattern <- Body".

See also: binary_comp/2, binary_generator_body/1, binary_generator_pattern/1, list_comp/2.

Returns the body subtree of a generator node.

See also: binary_generator/2.

Returns the pattern subtree of a generator node.

See also: binary_generator/2.

Creates an abstract block expression. If Body is [B1, ..., Bn], the result represents "begin B1, ..., Bn end".

See also: block_expr_body/1.

Returns the list of body subtrees of a block_expr node.

See also: block_expr/1.

Creates an abstract case-expression. If Clauses is [C1, ..., Cn], the result represents "case Argument of C1; ...; Cn end". More exactly, if each Ci represents "(Pi) Gi -> Bi", then the result represents "case Argument of P1 G1 -> B1; ...; Pn Gn -> Bn end".

See also: case_expr_argument/1, case_expr_clauses/1, clause/3, cond_expr/1, if_expr/1.

Returns the argument subtree of a case_expr node.

See also: case_expr/2.

Returns the list of clause subtrees of a case_expr node.

See also: case_expr/2.

Creates an abstract catch-expression. The result represents "catch Expr".

See also: catch_expr_body/1.

Returns the body subtree of a catch_expr node.

See also: catch_expr/1.

Creates an abstract character literal. The result represents "$Name", where Name corresponds to Value.

Note: the literal corresponding to a particular character value is not uniquely defined. E.g., the character "a" can be written both as "$a" and "$\141", and a Tab character can be written as "$\11", "$\011" or "$\t".

See also: char_literal/1, char_value/1, is_char/2.

Returns the literal string represented by a char node. This includes the leading "$" character.

See also: char/1.

Returns the value represented by a char node.

See also: char/1.

Creates an abstract class qualifier. The result represents "Class:Body".

See also: class_qualifier_argument/1, class_qualifier_body/1, try_expr/4.

Returns the argument (the class) subtree of a class_qualifier node.

See also: class_qualifier/2.

Returns the body subtree of a class_qualifier node.

See also: class_qualifier/2.

Equivalent to clause([], Guard, Body).

Types:

Creates an abstract clause. If Patterns is [P1, ..., Pn] and Body is [B1, ..., Bm], then if Guard is none, the result represents "(P1, ..., Pn) -> B1, ..., Bm", otherwise, unless Guard is a list, the result represents "(P1, ..., Pn) when Guard -> B1, ..., Bm".

For simplicity, the Guard argument may also be any of the following:

• An empty list []. This is equivalent to passing none.

• A nonempty list [E1, ..., Ej] of syntax trees. This is equivalent to passing conjunction([E1, ..., Ej]).

• A nonempty list of lists of syntax trees [[E1_1, ..., E1_k1], ..., [Ej_1, ..., Ej_kj]], which is equivalent to passing disjunction([conjunction([E1_1, ..., E1_k1]), ..., conjunction([Ej_1, ..., Ej_kj])]).

See also: clause/2, clause_body/1, clause_guard/1, clause_patterns/1.

Return the list of body subtrees of a clause node.

See also: clause/3.

Returns the guard subtree of a clause node, if any. If Node represents "(P1, ..., Pn) when Guard -> B1, ..., Bm", Guard is returned. Otherwise, the result is none.

See also: clause/3.

Returns the list of pattern subtrees of a clause node.

See also: clause/3.

Equivalent to comment(none, Strings).

Types:

Creates an abstract comment with the given padding and text. If Strings is a (possibly empty) list ["Txt1", ..., "TxtN"], the result represents the source code text

      %Txt1
      ...
      %TxtN

Padding states the number of empty character positions to the left of the comment separating it horizontally from source code on the same line (if any). If Padding is none, a default positive number is used. If Padding is an integer less than 1, there should be no separating space. Comments are in themselves regarded as source program forms.

See also: comment/1, is_form/1.

Returns the amount of padding before the comment, or none. The latter means that a default padding may be used.

See also: comment/2.

Returns the lines of text of the abstract comment.

See also: comment/2.

Yields the most compact form for an abstract list skeleton. The result either represents "[E1, ..., En | Tail]", where Tail is not a list skeleton, or otherwise simply "[E1, ..., En]". Annotations on subtrees of Node that represent list skeletons may be lost, but comments will be propagated to the result. Returns Node itself if Node does not represent a list skeleton.

See also: list/2, normalize_list/1.

Returns the Erlang term represented by a syntax tree. Evaluation fails with reason badarg if Node does not represent a literal term.

Note: Currently, the set of syntax trees which have a concrete representation is larger than the set of trees which can be built using the function abstract/1. An abstract character will be concretised as an integer, while abstract/1 does not at present yield an abstract character for any input. (Use the char/1 function to explicitly create an abstract character.)

See also: abstract/1, char/1, is_literal/1.

Creates an abstract cond-expression. If Clauses is [C1, ..., Cn], the result represents "cond C1; ...; Cn end". More exactly, if each Ci represents "() Ei -> Bi", then the result represents "cond E1 -> B1; ...; En -> Bn end".

See also: case_expr/2, clause/3, cond_expr_clauses/1.

Returns the list of clause subtrees of a cond_expr node.

See also: cond_expr/1.

Creates an abstract conjunction. If List is [E1, ..., En], the result represents "E1, ..., En".

See also: conjunction_body/1, disjunction/1.

Returns the list of body subtrees of a conjunction node.

See also: conjunction/1.

"Optimising" list skeleton cons operation. Creates an abstract list skeleton whose first element is Head and whose tail corresponds to Tail. This is similar to list([Head], Tail), except that Tail may not be none, and that the result does not necessarily represent exactly "[Head | Tail]", but may depend on the Tail subtree. E.g., if Tail represents [X, Y], the result may represent "[Head, X, Y]", rather than "[Head | [X, Y]]". Annotations on Tail itself may be lost if Tail represents a list skeleton, but comments on Tail are propagated to the result.

See also: list/2, list_head/1, list_tail/1.

Copies the list of user annotations from Source to Target.

Note: this is equivalent to set_ann(Target, get_ann(Source)), but potentially more efficient.

See also: get_ann/1, set_ann/2.

Copies the attributes from Source to Target.

Note: this is equivalent to set_attrs(Target, get_attrs(Source)), but potentially more efficient.

See also: get_attrs/1, set_attrs/2.

Copies the pre- and postcomments from Source to Target.

Note: This is equivalent to set_postcomments(set_precomments(Target, get_precomments(Source)), get_postcomments(Source)), but potentially more efficient.

See also: comment/2, get_postcomments/1, get_precomments/1, set_postcomments/2, set_precomments/2.

Copies the position information from Source to Target.

This is equivalent to set_pos(Target, get_pos(Source)), but potentially more efficient.

See also: get_pos/1, set_pos/2.

For special purposes only. Returns the associated data of a syntax tree node. Evaluation fails with reason badarg if is_tree(Node) does not yield true.

See also: tree/2.

Creates an abstract disjunction. If List is [E1, ..., En], the result represents "E1; ...; En".

See also: conjunction/1, disjunction_body/1.

Returns the list of body subtrees of a disjunction node.

See also: disjunction/1.

Creates an abstract end-of-file marker. This represents the end of input when reading a sequence of source code forms. An end-of-file marker is itself regarded as a source code form (namely, the last in any sequence in which it occurs). It has no defined lexical form.

Note: this is retained only for backwards compatibility with existing parsers and tools.

See also: error_marker/1, is_form/1, warning_marker/1.

Creates an abstract error marker. The result represents an occurrence of an error in the source code, with an associated Erlang I/O ErrorInfo structure given by Error (see module io(3) for details). Error markers are regarded as source code forms, but have no defined lexical form.

Note: this is supported only for backwards compatibility with existing parsers and tools.

See also: eof_marker/0, error_marker_info/1, is_form/1, warning_marker/1.

Returns the ErrorInfo structure of an error_marker node.

See also: error_marker/1.

Flattens sublists of a form_list node. Returns Node with all subtrees of type form_list recursively expanded, yielding a single "flat" abstract form sequence.

See also: form_list/1.

Creates an abstract floating-point literal. The lexical representation is the decimal floating-point numeral of Value.

See also: float_literal/1, float_value/1.

Returns the numeral string represented by a float node.

See also: float/1.

Returns the value represented by a float node. Note that floating-point values should usually not be compared for equality.

See also: float/1.

Creates an abstract sequence of "source code forms". If Forms is [F1, ..., Fn], where each Fi is a form (cf. is_form/1, the result represents

      F1
      ...
      Fn

where the Fi are separated by one or more line breaks. A node of type form_list is itself regarded as a source code form; cf. flatten_form_list/1.

Note: this is simply a way of grouping source code forms as a single syntax tree, usually in order to form an Erlang module definition.

See also: flatten_form_list/1, form_list_elements/1, is_form/1.

Returns the list of subnodes of a form_list node.

See also: form_list/1.

Creates an abstract fun-expression. If Clauses is [C1, ..., Cn], the result represents "fun C1; ...; Cn end". More exactly, if each Ci represents "(Pi1, ..., Pim) Gi -> Bi", then the result represents "fun (P11, ..., P1m) G1 -> B1; ...; (Pn1, ..., Pnm) Gn -> Bn end".

See also: fun_expr_arity/1, fun_expr_clauses/1.

Returns the arity of a fun_expr node. The result is the number of parameter patterns in the first clause of the fun-expression; subsequent clauses are ignored.

An exception is thrown if fun_expr_clauses(Node) returns an empty list, or if the first element of that list is not a syntax tree C of type clause such that clause_patterns(C) is a nonempty list.

See also: clause/3, clause_patterns/1, fun_expr/1, fun_expr_clauses/1.

Returns the list of clause subtrees of a fun_expr node.

See also: fun_expr/1.

Creates an abstract function definition. If Clauses is [C1, ..., Cn], the result represents "Name C1; ...; Name Cn.". More exactly, if each Ci represents "(Pi1, ..., Pim) Gi -> Bi", then the result represents "Name(P11, ..., P1m) G1 -> B1; ...; Name(Pn1, ..., Pnm) Gn -> Bn.". Function definitions are source code forms.

See also: function_arity/1, function_clauses/1, function_name/1, is_form/1, rule/2.

Returns the arity of a function node. The result is the number of parameter patterns in the first clause of the function; subsequent clauses are ignored.

An exception is thrown if function_clauses(Node) returns an empty list, or if the first element of that list is not a syntax tree C of type clause such that clause_patterns(C) is a nonempty list.

See also: clause/3, clause_patterns/1, function/2, function_clauses/1.

Returns the list of clause subtrees of a function node.

See also: function/2.

Returns the name subtree of a function node.

See also: function/2.

Creates an abstract generator. The result represents "Pattern <- Body".

See also: binary_comp/2, generator_body/1, generator_pattern/1, list_comp/2.

Returns the body subtree of a generator node.

See also: generator/2.

Returns the pattern subtree of a generator node.

See also: generator/2.

Returns the list of user annotations associated with a syntax tree node. For a newly created node, this is the empty list. The annotations may be any terms.

See also: get_attrs/1, set_ann/2.

Returns a representation of the attributes associated with a syntax tree node. The attributes are all the extra information that can be attached to a node. Currently, this includes position information, source code comments, and user annotations. The result of this function cannot be inspected directly; only attached to another node (cf. set_attrs/2).

For accessing individual attributes, see get_pos/1, get_ann/1, get_precomments/1 and get_postcomments/1.

See also: get_ann/1, get_pos/1, get_postcomments/1, get_precomments/1, set_attrs/2.

Returns the position information associated with Node. This is usually a nonnegative integer (indicating the source code line number), but may be any term. By default, all new tree nodes have their associated position information set to the integer zero.

See also: get_attrs/1, set_pos/2.

Returns the associated post-comments of a node. This is a possibly empty list of abstract comments, in top-down textual order. When the code is formatted, post-comments are typically displayed to the right of and/or below the node. For example:

          {foo, X, Y}     % Post-comment of tuple

If possible, the comment should be moved past any following separator characters on the same line, rather than placing the separators on the following line. E.g.:

          foo([X | Xs], Y) ->
              foo(Xs, bar(X));     % Post-comment of 'bar(X)' node
           ...

(where the comment is moved past the rightmost ")" and the ";").

See also: comment/2, get_attrs/1, get_precomments/1, set_postcomments/2.

Returns the associated pre-comments of a node. This is a possibly empty list of abstract comments, in top-down textual order. When the code is formatted, pre-comments are typically displayed directly above the node. For example:

          % Pre-comment of function
          foo(X) -> {bar, X}.

If possible, the comment should be moved before any preceding separator characters on the same line. E.g.:

          foo([X | Xs]) ->
              % Pre-comment of 'bar(X)' node
              [bar(X) | foo(Xs)];
          ...

(where the comment is moved before the "[").

See also: comment/2, get_attrs/1, get_postcomments/1, set_precomments/2.

Yields false if the node has no associated comments, and true otherwise.

Note: This is equivalent to (get_precomments(Node) == []) and (get_postcomments(Node) == []), but potentially more efficient.

See also: get_postcomments/1, get_precomments/1, remove_comments/1.

Creates an abstract if-expression. If Clauses is [C1, ..., Cn], the result represents "if C1; ...; Cn end". More exactly, if each Ci represents "() Gi -> Bi", then the result represents "if G1 -> B1; ...; Gn -> Bn end".

See also: case_expr/2, clause/3, if_expr_clauses/1.

Returns the list of clause subtrees of an if_expr node.

See also: if_expr/1.

Creates an abstract "implicit fun" expression. The result represents "fun Name". Name should represent either F/A or M:F/A

See also: arity_qualifier/2, implicit_fun/2, implicit_fun/3, implicit_fun_name/1, module_qualifier/2.

Creates an abstract "implicit fun" expression. If Arity is none, this is equivalent to implicit_fun(Name), otherwise it is equivalent to implicit_fun(arity_qualifier(Name, Arity)).

(This is a utility function.)

See also: implicit_fun/1, implicit_fun/3.

Creates an abstract module-qualified "implicit fun" expression. If Module is none, this is equivalent to implicit_fun(Name, Arity), otherwise it is equivalent to implicit_fun(module_qualifier(Module, arity_qualifier(Name, Arity)).

(This is a utility function.)

See also: implicit_fun/1, implicit_fun/2.

Returns the name subtree of an implicit_fun node.

Note: if Node represents "fun N/A" or "fun M:N/A", then the result is the subtree representing "N/A" or "M:N/A", respectively.

See also: arity_qualifier/2, implicit_fun/1, module_qualifier/2.

Creates an abstract infix operator expression. The result represents "Left Operator Right".

See also: infix_expr_left/1, infix_expr_operator/1, infix_expr_right/1, prefix_expr/2.

Returns the left argument subtree of an infix_expr node.

See also: infix_expr/3.

Returns the operator subtree of an infix_expr node.

See also: infix_expr/3.

Returns the right argument subtree of an infix_expr node.

See also: infix_expr/3.

Creates an abstract integer literal. The lexical representation is the canonical decimal numeral of Value.

See also: integer_literal/1, integer_value/1, is_integer/2.

Returns the numeral string represented by an integer node.

See also: integer/1.

Returns the value represented by an integer node.

See also: integer/1.

Returns true if Node has type atom and represents Value, otherwise false.

See also: atom/1.

Returns true if Node has type char and represents Value, otherwise false.

See also: char/1.

Returns true if Node is a syntax tree representing a so-called "source code form", otherwise false. Forms are the Erlang source code units which, placed in sequence, constitute an Erlang program. Current form types are:

attribute comment error_marker eof_marker form_list function rule warning_marker text

See also: attribute/2, comment/2, eof_marker/0, error_marker/1, form_list/1, function/2, rule/2, type/1, warning_marker/1.

Returns true if Node has type integer and represents Value, otherwise false.

See also: integer/1.

Returns true if Node is a leaf node, otherwise false. The currently recognised leaf node types are:

atom char comment eof_marker error_marker float integer nil operator string text underscore variable warning_marker

A node of type tuple is a leaf node if and only if its arity is zero.

Note: not all literals are leaf nodes, and vice versa. E.g., tuples with nonzero arity and nonempty lists may be literals, but are not leaf nodes. Variables, on the other hand, are leaf nodes but not literals.

See also: is_literal/1, type/1.

Returns true if Node has type list or nil, otherwise false.

See also: list/2, nil/0.

Returns true if Node represents a literal term, otherwise false. This function returns true if and only if the value of concrete(Node) is defined.

See also: abstract/1, concrete/1.

Returns true if Node represents a proper list, and false otherwise. A proper list is a list skeleton either on the form "[]" or "[E1, ..., En]", or "[... | Tail]" where recursively Tail also represents a proper list.

Note: Since Node is a syntax tree, the actual run-time values corresponding to its subtrees may often be partially or completely unknown. Thus, if Node represents e.g. "[... | Ns]" (where Ns is a variable), then the function will return false, because it is not known whether Ns will be bound to a list at run-time. If Node instead represents e.g. "[1, 2, 3]" or "[A | []]", then the function will return true.

See also: list/2.

Returns true if Node has type string and represents Value, otherwise false.

See also: string/1.

For special purposes only. Returns true if Tree is an abstract syntax tree and false otherwise.

Note: this function yields false for all "old-style" erl_parse-compatible "parse trees".

See also: tree/2.

Appends the comments of Source to the current comments of Target.

Note: This is equivalent to add_postcomments(get_postcomments(Source), add_precomments(get_precomments(Source), Target)), but potentially more efficient.

See also: add_postcomments/2, add_precomments/2, comment/2, get_postcomments/1, get_precomments/1.

Equivalent to list(List, none).

Types:

Constructs an abstract list skeleton. The result has type list or nil. If List is a nonempty list [E1, ..., En], the result has type list and represents either "[E1, ..., En]", if Tail is none, or otherwise "[E1, ..., En | Tail]". If List is the empty list, Tail must be none, and in that case the result has type nil and represents "[]" (cf. nil/0).

The difference between lists as semantic objects (built up of individual "cons" and "nil" terms) and the various syntactic forms for denoting lists may be bewildering at first. This module provides functions both for exact control of the syntactic representation as well as for the simple composition and deconstruction in terms of cons and head/tail operations.

Note: in list(Elements, none), the "nil" list terminator is implicit and has no associated information (cf. get_attrs/1), while in the seemingly equivalent list(Elements, Tail) when Tail has type nil, the list terminator subtree Tail may have attached attributes such as position, comments, and annotations, which will be preserved in the result.

See also: compact_list/1, cons/2, get_attrs/1, is_list_skeleton/1, is_proper_list/1, list/1, list_elements/1, list_head/1, list_length/1, list_prefix/1, list_suffix/1, list_tail/1, nil/0, normalize_list/1.

Creates an abstract list comprehension. If Body is [E1, ..., En], the result represents "[Template || E1, ..., En]".

See also: generator/2, list_comp_body/1, list_comp_template/1.

Returns the list of body subtrees of a list_comp node.

See also: list_comp/2.

Returns the template subtree of a list_comp node.

See also: list_comp/2.

Returns the list of element subtrees of a list skeleton. Node must represent a proper list. E.g., if Node represents "[X1, X2 | [X3, X4 | []]", then list_elements(Node) yields the list [X1, X2, X3, X4].

See also: is_proper_list/1, list/2.

Returns the head element subtree of a list node. If Node represents "[Head ...]", the result will represent "Head".

See also: cons/2, list/2, list_tail/1.

Returns the number of element subtrees of a list skeleton. Node must represent a proper list. E.g., if Node represents "[X1 | [X2, X3 | [X4, X5, X6]]]", then list_length(Node) returns the integer 6.

Note: this is equivalent to length(list_elements(Node)), but potentially more efficient.

See also: is_proper_list/1, list/2, list_elements/1.

Returns the prefix element subtrees of a list node. If Node represents "[E1, ..., En]" or "[E1, ..., En | Tail]", the returned value is [E1, ..., En].

See also: list/2.

Returns the suffix subtree of a list node, if one exists. If Node represents "[E1, ..., En | Tail]", the returned value is Tail, otherwise, i.e., if Node represents "[E1, ..., En]", none is returned.

Note that even if this function returns some Tail that is not none, the type of Tail can be nil, if the tail has been given explicitly, and the list skeleton has not been compacted (cf. compact_list/1).

See also: compact_list/1, list/2, nil/0.

Returns the tail of a list node. If Node represents a single-element list "[E]", then the result has type nil, representing "[]". If Node represents "[E1, E2 ...]", the result will represent "[E2 ...]", and if Node represents "[Head | Tail]", the result will represent "Tail".

See also: cons/2, list/2, list_head/1.

Equivalent to macro(Name, none).

Types:

Creates an abstract macro application. If Arguments is none, the result represents "?Name", otherwise, if Arguments is [A1, ..., An], the result represents "?Name(A1, ..., An)".

Notes: if Arguments is the empty list, the result will thus represent "?Name()", including a pair of matching parentheses.

The only syntactical limitation imposed by the preprocessor on the arguments to a macro application (viewed as sequences of tokens) is that they must be balanced with respect to parentheses, brackets, begin ... end, case ... end, etc. The text node type can be used to represent arguments which are not regular Erlang constructs.

See also: macro/1, macro_arguments/1, macro_name/1, text/1.

Returns the list of argument subtrees of a macro node, if any. If Node represents "?Name", none is returned. Otherwise, if Node represents "?Name(A1, ..., An)", [A1, ..., An] is returned.

See also: macro/2.

Returns the name subtree of a macro node.

See also: macro/2.

Creates a syntax tree with the given type and subtrees. Type must be a node type name (cf. type/1) that does not denote a leaf node type (cf. is_leaf/1). Groups must be a nonempty list of groups of syntax trees, representing the subtrees of a node of the given type, in left-to-right order as they would occur in the printed program text, grouped by category as done by subtrees/1.

The result of copy_attrs(Node, make_tree(type(Node), subtrees(Node))) (cf. update_tree/2) represents the same source code text as the original Node, assuming that subtrees(Node) yields a nonempty list. However, it does not necessarily have the same data representation as Node.

See also: copy_attrs/2, is_leaf/1, subtrees/1, type/1, update_tree/2.

Creates an abstract match-expression. The result represents "Pattern = Body".

See also: match_expr_body/1, match_expr_pattern/1.

Returns the body subtree of a match_expr node.

See also: match_expr/2.

Returns the pattern subtree of a match_expr node.

See also: match_expr/2.

Creates a meta-representation of a syntax tree. The result represents an Erlang expression "MetaTree" which, if evaluated, will yield a new syntax tree representing the same source code text as Tree (although the actual data representation may be different). The expression represented by MetaTree is implementation independent with regard to the data structures used by the abstract syntax tree implementation. Comments attached to nodes of Tree will be preserved, but other attributes are lost.

Any node in Tree whose node type is variable (cf. type/1), and whose list of annotations (cf. get_ann/1) contains the atom meta_var, will remain unchanged in the resulting tree, except that exactly one occurrence of meta_var is removed from its annotation list.

The main use of the function meta/1 is to transform a data structure Tree, which represents a piece of program code, into a form that is representation independent when printed. E.g., suppose Tree represents a variable named "V". Then (assuming a function print/1 for printing syntax trees), evaluating print(abstract(Tree)) - simply using abstract/1 to map the actual data structure onto a syntax tree representation - would output a string that might look something like "{tree, variable, ..., "V", ...}", which is obviously dependent on the implementation of the abstract syntax trees. This could e.g. be useful for caching a syntax tree in a file. However, in some situations like in a program generator generator (with two "generator"), it may be unacceptable. Using print(meta(Tree)) instead would output a representation independent syntax tree generating expression; in the above case, something like "erl_syntax:variable("V")".

See also: abstract/1, get_ann/1, type/1.

Creates an abstract module qualifier. The result represents "Module:Body".

See also: module_qualifier_argument/1, module_qualifier_body/1.

Returns the argument (the module) subtree of a module_qualifier node.

See also: module_qualifier/2.

Returns the body subtree of a module_qualifier node.

See also: module_qualifier/2.

Creates an abstract empty list. The result represents "[]". The empty list is traditionally called "nil".

See also: is_list_skeleton/1, list/2.

Expands an abstract list skeleton to its most explicit form. If Node represents "[E1, ..., En | Tail]", the result represents "[E1 | ... [En | Tail1] ... ]", where Tail1 is the result of normalize_list(Tail). If Node represents "[E1, ..., En]", the result simply represents "[E1 | ... [En | []] ... ]". If Node does not represent a list skeleton, Node itself is returned.

See also: compact_list/1, list/2.

Types:

Creates an abstract operator. The name of the operator is the character sequence represented by Name. This is analogous to the print name of an atom, but an operator is never written within single-quotes; e.g., the result of operator('++') represents "++" rather than "'++'".

See also: atom/1, operator_literal/1, operator_name/1.

Returns the literal string represented by an operator node. This is simply the operator name as a string.

See also: operator/1.

Returns the name of an operator node. Note that the name is returned as an atom.

See also: operator/1.

Creates an abstract parenthesised expression. The result represents "(Body)", independently of the context.

See also: parentheses_body/1.

Returns the body subtree of a parentheses node.

See also: parentheses/1.

Creates an abstract prefix operator expression. The result represents "Operator Argument".

See also: infix_expr/3, prefix_expr_argument/1, prefix_expr_operator/1.

Returns the argument subtree of a prefix_expr node.

See also: prefix_expr/2.

Returns the operator subtree of a prefix_expr node.

See also: prefix_expr/2.

Creates an abstract qualified name. The result represents "S1.S2. ... .Sn", if Segments is [S1, S2, ..., Sn].

See also: qualified_name_segments/1.

Returns the list of name segments of a qualified_name node.

See also: qualified_name/1.

Creates an abstract Mnemosyne query expression. The result represents "query Body end".

See also: query_expr_body/1, record_access/2, rule/2.

Returns the body subtree of a query_expr node.

See also: query_expr/1.

Equivalent to receive_expr(Clauses, none, []).

Types:

Creates an abstract receive-expression. If Timeout is none, the result represents "receive C1; ...; Cn end" (the Action argument is ignored). Otherwise, if Clauses is [C1, ..., Cn] and Action is [A1, ..., Am], the result represents "receive C1; ...; Cn after Timeout -> A1, ..., Am end". More exactly, if each Ci represents "(Pi) Gi -> Bi", then the result represents "receive P1 G1 -> B1; ...; Pn Gn -> Bn ... end".

Note that in Erlang, a receive-expression must have at least one clause if no timeout part is specified.

See also: case_expr/2, clause/3, receive_expr/1, receive_expr_action/1, receive_expr_clauses/1, receive_expr_timeout/1.

Returns the list of action body subtrees of a receive_expr node. If Node represents "receive C1; ...; Cn end", this is the empty list.

See also: receive_expr/3.

Types:

Returns the list of clause subtrees of a receive_expr node.

See also: receive_expr/3.

Types:

Returns the timeout subtree of a receive_expr node, if any. If Node represents "receive C1; ...; Cn end", none is returned. Otherwise, if Node represents "receive C1; ...; Cn after Timeout -> ... end", Timeout is returned.

See also: receive_expr/3.

Equivalent to record_access(Argument, none, Field).

Types:

Creates an abstract record field access expression. If Type is not none, the result represents "Argument#Type.Field".

If Type is none, the result represents "Argument.Field". This is a special form only allowed within Mnemosyne queries.

See also: query_expr/1, record_access/2, record_access_argument/1, record_access_field/1, record_access_type/1, record_expr/3.

Returns the argument subtree of a record_access node.

See also: record_access/3.

Returns the field subtree of a record_access node.

See also: record_access/3.

Returns the type subtree of a record_access node, if any. If Node represents "Argument.Field", none is returned, otherwise if Node represents "Argument#Type.Field", Type is returned.

See also: record_access/3.

Equivalent to record_expr(none, Type, Fields).

Types:

Creates an abstract record expression. If Fields is [F1, ..., Fn], then if Argument is none, the result represents "#Type{F1, ..., Fn}", otherwise it represents "Argument#Type{F1, ..., Fn}".

See also: record_access/3, record_expr/2, record_expr_argument/1, record_expr_fields/1, record_expr_type/1, record_field/2, record_index_expr/2.

Returns the argument subtree of a record_expr node, if any. If Node represents "#Type{...}", none is returned. Otherwise, if Node represents "Argument#Type{...}", Argument is returned.

See also: record_expr/3.

Returns the list of field subtrees of a record_expr node.

See also: record_expr/3.

Returns the type subtree of a record_expr node.

See also: record_expr/3.

Equivalent to record_field(Name, none).

Types:

Creates an abstract record field specification. If Value is none, the result represents simply "Name", otherwise it represents "Name = Value".

See also: record_expr/3, record_field_name/1, record_field_value/1.

Returns the name subtree of a record_field node.

See also: record_field/2.

Returns the value subtree of a record_field node, if any. If Node represents "Name", none is returned. Otherwise, if Node represents "Name = Value", Value is returned.

See also: record_field/2.

Creates an abstract record field index expression. The result represents "#Type.Field".

(Note: the function name record_index/2 is reserved by the Erlang compiler, which is why that name could not be used for this constructor.)

See also: record_expr/3, record_index_expr_field/1, record_index_expr_type/1.

Returns the field subtree of a record_index_expr node.

See also: record_index_expr/2.

Returns the type subtree of a record_index_expr node.

See also: record_index_expr/2.

Clears the associated comments of Node.

Note: This is equivalent to set_precomments(set_postcomments(Node, []), []), but potentially more efficient.

See also: set_postcomments/2, set_precomments/2.

Returns an erl_parse-compatible representation of a syntax tree, if possible. If Tree represents a well-formed Erlang program or expression, the conversion should work without problems. Typically, is_tree/1 yields true if conversion failed (i.e., the result is still an abstract syntax tree), and false otherwise.

The is_tree/1 test is not completely foolproof. For a few special node types (e.g. arity_qualifier), if such a node occurs in a context where it is not expected, it will be left unchanged as a non-reverted subtree of the result. This can only happen if Tree does not actually represent legal Erlang code.

See also: erl_parse(3), revert_forms/1.

Types:

Reverts a sequence of Erlang source code forms. The sequence can be given either as a form_list syntax tree (possibly nested), or as a list of "program form" syntax trees. If successful, the corresponding flat list of erl_parse-compatible syntax trees is returned (cf. revert/1). If some program form could not be reverted, {error, Form} is thrown. Standalone comments in the form sequence are discarded.

See also: form_list/1, is_form/1, revert/1.

Creates an abstract Mnemosyne rule. If Clauses is [C1, ..., Cn], the results represents "Name C1; ...; Name Cn.". More exactly, if each Ci represents "(Pi1, ..., Pim) Gi -> Bi", then the result represents "Name(P11, ..., P1m) G1 :- B1; ...; Name(Pn1, ..., Pnm) Gn :- Bn.". Rules are source code forms.

See also: function/2, is_form/1, rule_arity/1, rule_clauses/1, rule_name/1.

Returns the arity of a rule node. The result is the number of parameter patterns in the first clause of the rule; subsequent clauses are ignored.

An exception is thrown if rule_clauses(Node) returns an empty list, or if the first element of that list is not a syntax tree C of type clause such that clause_patterns(C) is a nonempty list.

See also: clause/3, clause_patterns/1, rule/2, rule_clauses/1.

Returns the list of clause subtrees of a rule node.

See also: rule/2.

Returns the name subtree of a rule node.

See also: rule/2.

Sets the list of user annotations of Node to Annotations.

See also: add_ann/2, copy_ann/2, get_ann/1.

Sets the attributes of Node to Attributes.

See also: copy_attrs/2, get_attrs/1.

Sets the position information of Node to Pos.

See also: copy_pos/2, get_pos/1.

Sets the post-comments of Node to Comments. Comments should be a possibly empty list of abstract comments, in top-down textual order

See also: add_postcomments/2, comment/2, copy_comments/2, get_postcomments/1, join_comments/2, remove_comments/1, set_precomments/2.

Sets the pre-comments of Node to Comments. Comments should be a possibly empty list of abstract comments, in top-down textual order.

See also: add_precomments/2, comment/2, copy_comments/2, get_precomments/1, join_comments/2, remove_comments/1, set_postcomments/2.

Creates an abstract size qualifier. The result represents "Body:Size".

See also: size_qualifier_argument/1, size_qualifier_body/1.

Returns the argument subtree (the size) of a size_qualifier node.

See also: size_qualifier/2.

Returns the body subtree of a size_qualifier node.

See also: size_qualifier/2.

Creates an abstract string literal. The result represents "Text" (including the surrounding double-quotes), where Text corresponds to the sequence of characters in Value, but not representing a specific string literal. E.g., the result of string("x\ny") represents any and all of "x\ny", "x\12y", "x\012y" and "x\^Jy"; cf. char/1.

See also: char/1, is_string/2, string_literal/1, string_value/1.

Returns the literal string represented by a string node. This includes surrounding double-quote characters.

See also: string/1.

Returns the value represented by a string node.

See also: string/1.

Returns the grouped list of all subtrees of a syntax tree. If Node is a leaf node (cf. is_leaf/1), this is the empty list, otherwise the result is always a nonempty list, containing the lists of subtrees of Node, in left-to-right order as they occur in the printed program text, and grouped by category. Often, each group contains only a single subtree.

Depending on the type of Node, the size of some groups may be variable (e.g., the group consisting of all the elements of a tuple), while others always contain the same number of elements - usually exactly one (e.g., the group containing the argument expression of a case-expression). Note, however, that the exact structure of the returned list (for a given node type) should in general not be depended upon, since it might be subject to change without notice.

The function subtrees/1 and the constructor functions make_tree/2 and update_tree/2 can be a great help if one wants to traverse a syntax tree, visiting all its subtrees, but treat nodes of the tree in a uniform way in most or all cases. Using these functions makes this simple, and also assures that your code is not overly sensitive to extensions of the syntax tree data type, because any node types not explicitly handled by your code can be left to a default case.

For example:

    postorder(F, Tree) ->
        F(case subtrees(Tree) of
            [] -> Tree;
            List -> update_tree(Tree,
                                [[postorder(F, Subtree)
                                  || Subtree <- Group]
                                 || Group <- List])
          end).
  

maps the function F on Tree and all its subtrees, doing a post-order traversal of the syntax tree. (Note the use of update_tree/2 to preserve node attributes.) For a simple function like:

    f(Node) ->
        case type(Node) of
            atom -> atom("a_" ++ atom_name(Node));
            _ -> Node
        end.
  

the call postorder(fun f/1, Tree) will yield a new representation of Tree in which all atom names have been extended with the prefix "a_", but nothing else (including comments, annotations and line numbers) has been changed.

See also: copy_attrs/2, is_leaf/1, make_tree/2, type/1.

Creates an abstract piece of source code text. The result represents exactly the sequence of characters in String. This is useful in cases when one wants full control of the resulting output, e.g., for the appearance of floating-point numbers or macro definitions.

See also: text_string/1.

Returns the character sequence represented by a text node.

See also: text/1.

Equivalent to tree(Type, []).

For special purposes only. Creates an abstract syntax tree node with type tag Type and associated data Data.

This function and the related is_tree/1 and data/1 provide a uniform way to extend the set of erl_parse node types. The associated data is any term, whose format may depend on the type tag.

Notes:

• Any nodes created outside of this module must have type tags distinct from those currently defined by this module; see type/1 for a complete list.

• The type tag of a syntax tree node may also be used as a primary tag by the erl_parse representation; in that case, the selector functions for that node type must handle both the abstract syntax tree and the erl_parse form. The function type(T) should return the correct type tag regardless of the representation of T, so that the user sees no difference between erl_syntax and erl_parse nodes.

See also: data/1, is_tree/1, type/1.

Equivalent to try_expr(Body, [], [], After).

Equivalent to try_expr(Body, [], Handlers).

Equivalent to try_expr(Body, Clauses, Handlers, []).

Creates an abstract try-expression. If Body is [B1, ..., Bn], Clauses is [C1, ..., Cj], Handlers is [H1, ..., Hk], and After is [A1, ..., Am], the result represents "try B1, ..., Bn of C1; ...; Cj catch H1; ...; Hk after A1, ..., Am end". More exactly, if each Ci represents "(CPi) CGi -> CBi", and each Hi represents "(HPi) HGi -> HBi", then the result represents "try B1, ..., Bn of CP1 CG1 -> CB1; ...; CPj CGj -> CBj catch HP1 HG1 -> HB1; ...; HPk HGk -> HBk after A1, ..., Am end"; cf. case_expr/2. If Clauses is the empty list, the of ... section is left out. If After is the empty list, the after ... section is left out. If Handlers is the empty list, and After is nonempty, the catch ... section is left out.

See also: case_expr/2, class_qualifier/2, clause/3, try_after_expr/2, try_expr/2, try_expr/3, try_expr_after/1, try_expr_body/1, try_expr_clauses/1, try_expr_handlers/1.

Returns the list of "after" subtrees of a try_expr node.

See also: try_expr/4.

Returns the list of body subtrees of a try_expr node.

See also: try_expr/4.

Returns the list of case-clause subtrees of a try_expr node. If Node represents "try Body catch H1; ...; Hn end", the result is the empty list.

See also: try_expr/4.

Returns the list of handler-clause subtrees of a try_expr node.

See also: try_expr/4.

Creates an abstract tuple. If Elements is [X1, ..., Xn], the result represents "{X1, ..., Xn}".

Note: The Erlang language has distinct 1-tuples, i.e., {X} is always distinct from X itself.

See also: tuple_elements/1, tuple_size/1.

Returns the list of element subtrees of a tuple node.

See also: tuple/1.

Returns the number of elements of a tuple node.

Note: this is equivalent to length(tuple_elements(Node)), but potentially more efficient.

See also: tuple/1, tuple_elements/1.

Returns the type tag of Node. If Node does not represent a syntax tree, evaluation fails with reason badarg. Node types currently defined by this module are:

application arity_qualifier atom attribute binary binary_field block_expr case_expr catch_expr char class_qualifier clause comment cond_expr conjunction disjunction eof_marker error_marker float form_list fun_expr function generator if_expr implicit_fun infix_expr integer list list_comp macro match_expr module_qualifier nil operator parentheses prefix_expr qualified_name query_expr receive_expr record_access record_expr record_field record_index_expr rule size_qualifier string text try_expr tuple underscore variable warning_marker

The user may (for special purposes) create additional nodes with other type tags, using the tree/2 function.

Note: The primary constructor functions for a node type should always have the same name as the node type itself.

See also: application/3, arity_qualifier/2, atom/1, attribute/2, binary/1, binary_field/2, block_expr/1, case_expr/2, catch_expr/1, char/1, class_qualifier/2, clause/3, comment/2, cond_expr/1, conjunction/1, disjunction/1, eof_marker/0, error_marker/1, float/1, form_list/1, fun_expr/1, function/2, generator/2, if_expr/1, implicit_fun/2, infix_expr/3, integer/1, list/2, list_comp/2, macro/2, match_expr/2, module_qualifier/2, nil/0, operator/1, parentheses/1, prefix_expr/2, qualified_name/1, query_expr/1, receive_expr/3, record_access/3, record_expr/2, record_field/2, record_index_expr/2, rule/2, size_qualifier/2, string/1, text/1, tree/2, try_expr/3, tuple/1, underscore/0, variable/1, warning_marker/1.

Creates an abstract universal pattern ("_"). The lexical representation is a single underscore character. Note that this is not a variable, lexically speaking.

See also: variable/1.

Creates a syntax tree with the same type and attributes as the given tree. This is equivalent to copy_attrs(Node, make_tree(type(Node), Groups)).

See also: copy_attrs/2, make_tree/2, type/1.

Types:

Creates an abstract variable with the given name. Name may be any atom or string that represents a lexically valid variable name, but not a single underscore character; cf. underscore/0.

Note: no checking is done whether the character sequence represents a proper variable name, i.e., whether or not its first character is an uppercase Erlang character, or whether it does not contain control characters, whitespace, etc.

See also: underscore/0, variable_literal/1, variable_name/1.

Returns the name of a variable node as a string.

See also: variable/1.

Returns the name of a variable node as an atom.

See also: variable/1.

Creates an abstract warning marker. The result represents an occurrence of a possible problem in the source code, with an associated Erlang I/O ErrorInfo structure given by Error (see module io(3) for details). Warning markers are regarded as source code forms, but have no defined lexical form.

Note: this is supported only for backwards compatibility with existing parsers and tools.

See also: eof_marker/0, error_marker/1, is_form/1, warning_marker_info/1.

Returns the ErrorInfo structure of a warning_marker node.

See also: warning_marker/1.