4 The Abstract Format
This document describes the standard representation of parse trees for Erlang
programs as Erlang terms. This representation is known as the abstract format.
Functions dealing with such parse trees are compile:forms/[1,2]
and functions in the modules
epp,
erl_eval,
erl_lint,
erl_pp,
erl_parse,
and
io.
They are also used as input and output for parse transforms (see the module
compile).
We use the function Rep to denote the mapping from an Erlang source
construct C to its abstract format representation R, and write
R = Rep(C).
The word LINE below represents an integer, and denotes the
number of the line in the source file where the construction occurred.
Several instances of LINE in the same construction may denote
different lines.
Since operators are not terms in their own right, when operators are
mentioned below, the representation of an operator should be taken to
be the atom with a printname consisting of the same characters as the
operator.
4.1 Module declarations and forms
A module declaration consists of a sequence of forms that are either
function declarations or attributes.
-
If D is a module declaration consisting of the forms
F_1, ..., F_k, then
Rep(D) = [Rep(F_1), ..., Rep(F_k)].
-
If F is an attribute -module(Mod), then
Rep(F) = {attribute,LINE,module,Mod}.
-
If F is an attribute -export([Fun_1/A_1, ..., Fun_k/A_k]), then
Rep(F) = {attribute,LINE,export,[{Fun_1,A_1}, ..., {Fun_k,A_k}]}.
-
If F is an attribute -import(Mod,[Fun_1/A_1, ..., Fun_k/A_k]), then
Rep(F) = {attribute,LINE,import,{Mod,[{Fun_1,A_1}, ..., {Fun_k,A_k}]}}.
-
If F is an attribute -compile(Options), then
Rep(F) = {attribute,LINE,compile,Options}.
-
If F is an attribute -file(File,Line), then
Rep(F) = {attribute,LINE,file,{File,Line}}.
-
If F is a record declaration -record(Name,{V_1, ..., V_k}), then
Rep(F) =
{attribute,LINE,record,{Name,[Rep(V_1), ..., Rep(V_k)]}}. For Rep(V), see below.
-
If F is a wild attribute -A(T), then
Rep(F) = {attribute,LINE,A,T}.
-
If F is a function declaration Name Fc_1 ; ... ; Name Fc_k,
where each Fc_i is a function clause with a
pattern sequence of the same length Arity, then
Rep(F) = {function,LINE,Name,Arity,[Rep(Fc_1), ...,Rep(Fc_k)]}.
4.1.1 Record fields
Each field in a record declaration may have an optional
explicit default initializer expression
-
If V is A, then
Rep(V) = {record_field,LINE,Rep(A)}.
-
If V is A = E, then
Rep(V) = {record_field,LINE,Rep(A),Rep(E)}.
4.1.2 Representation of parse errors and end of file
In addition to the representations of forms, the list that represents
a module declaration (as returned by functions in erl_parse and
epp) may contain tuples {error,E} and {warning,W}, denoting
syntactically incorrect forms and warnings, and {eof,LINE}, denoting an end
of stream encountered before a complete form had been parsed.
4.2 Atomic literals
There are five kinds of atomic literals, which are represented in the
same way in patterns, expressions and guards:
-
If L is an integer or character literal, then
Rep(L) = {integer,LINE,L}.
-
If L is a float literal, then
Rep(L) = {float,LINE,L}.
-
If L is a string literal consisting of the characters
C_1, ..., C_k, then
Rep(L) = {string,LINE,[C_1, ..., C_k]}.
-
If L is an atom literal, then
Rep(L) = {atom,LINE,L}.
Note that negative integer and float literals do not occur as such; they are
parsed as an application of the unary negation operator.
4.3 Patterns
If Ps is a sequence of patterns P_1, ..., P_k, then
Rep(Ps) = [Rep(P_1), ..., Rep(P_k)]. Such sequences occur as the
list of arguments to a function or fun.
Individual patterns are represented as follows:
-
If P is an atomic literal L, then Rep(P) = Rep(L).
-
If P is a compound pattern P_1 = P_2, then
Rep(P) = {match,LINE,Rep(P_1),Rep(P_2)}.
-
If P is a variable pattern V, then
Rep(P) = {var,LINE,A},
where A is an atom with a printname consisting of the same characters as
V.
-
If P is a universal pattern _, then
Rep(P) = {var,LINE,'_'}.
-
If P is a tuple pattern {P_1, ..., P_k}, then
Rep(P) = {tuple,LINE,[Rep(P_1), ..., Rep(P_k)]}.
-
If P is a nil pattern [], then
Rep(P) = {nil,LINE}.
-
If P is a cons pattern [P_h | P_t], then
Rep(P) = {cons,LINE,Rep(P_h),Rep(P_t)}.
-
If E is a binary pattern <<P_1:Size_1/TSL_1, ..., P_k:Size_k/TSL_k>>, then
Rep(E) = {bin,LINE,[{bin_element,LINE,Rep(P_1),Rep(Size_1),Rep(TSL_1)}, ..., {bin_element,LINE,Rep(P_k),Rep(Size_k),Rep(TSL_k)}]}.
For Rep(TSL), see below.
An omitted Size is represented by default. An omitted TSL
(type specifier list) is represented by default.
-
If P is P_1 Op P_2, where Op is a binary operator (this
is either an occurrence of ++ applied to a literal string or character
list, or an occurrence of an expression that can be evaluated to a number
at compile time),
then Rep(P) = {op,LINE,Op,Rep(P_1),Rep(P_2)}.
-
If P is Op P_0, where Op is a unary operator (this is an
occurrence of an expression that can be evaluated to a number at compile
time), then Rep(P) = {op,LINE,Op,Rep(P_0)}.
-
If P is a record pattern #Name{Field_1=P_1, ..., Field_k=P_k},
then Rep(P) =
{record,LINE,Name, [{record_field,LINE,Rep(Field_1),Rep(P_1)}, ..., {record_field,LINE,Rep(Field_k),Rep(P_k)}]}.
-
If P is #Name.Field, then
Rep(P) = {record_index,LINE,Name,Rep(Field)}.
-
If P is ( P_0 ), then
Rep(P) = Rep(P_0),
i.e., patterns cannot be distinguished from their bodies.
Note that every pattern has the same source form as some expression, and is
represented the same way as the corresponding expression.
4.4 Expressions
A body B is a sequence of expressions E_1, ..., E_k, and
Rep(B) = [Rep(E_1), ..., Rep(E_k)].
An expression E is one of the following alternatives:
-
If P is an atomic literal L, then
Rep(P) = Rep(L).
-
If E is P = E_0, then
Rep(E) = {match,LINE,Rep(P),Rep(E_0)}.
-
If E is a variable V, then
Rep(E) = {var,LINE,A},
where A is an atom with a printname consisting of the same
characters as V.
-
If E is a tuple skeleton {E_1, ..., E_k}, then
Rep(E) = {tuple,LINE,[Rep(E_1), ..., Rep(E_k)]}.
-
If E is [], then
Rep(E) = {nil,LINE}.
-
If E is a cons skeleton [E_h | E_t], then
Rep(E) = {cons,LINE,Rep(E_h),Rep(E_t)}.
-
If E is a binary constructor <<V_1:Size_1/TSL_1, ..., V_k:Size_k/TSL_k>>, then
Rep(E) = {bin,LINE,[{bin_element,LINE,Rep(V_1),Rep(Size_1),Rep(TSL_1)}, ..., {bin_element,LINE,Rep(V_k),Rep(Size_k),Rep(TSL_k)}]}.
For Rep(TSL), see below.
An omitted Size is represented by default. An omitted TSL
(type specifier list) is represented by default.
-
If E is E_1 Op E_2, where Op is a binary operator,
then Rep(E) = {op,LINE,Op,Rep(E_1),Rep(E_2)}.
-
If E is Op E_0, where Op is a unary operator, then
Rep(E) = {op,LINE,Op,Rep(E_0)}.
-
If E is #Name{Field_1=E_1, ..., Field_k=E_k}, then
Rep(E) =
{record,LINE,Name, [{record_field,LINE,Rep(Field_1),Rep(E_1)}, ..., {record_field,LINE,Rep(Field_k),Rep(E_k)}]}.
-
If E is E_0#Name{Field_1=E_1, ..., Field_k=E_k}, then
Rep(E) =
{record,LINE,Rep(E_0),Name, [{record_field,LINE,Rep(Field_1),Rep(E_1)}, ..., {record_field,LINE,Rep(Field_k),Rep(E_k)}]}.
-
If E is #Name.Field, then
Rep(E) = {record_index,LINE,Name,Rep(Field)}.
-
If E is E_0#Name.Field, then
Rep(E) = {record_field,LINE,Rep(E_0),Name,Rep(Field)}.
-
If E is catch E_0, then
Rep(E) = {'catch',LINE,Rep(E_0)}.
-
If E is E_0(E_1, ..., E_k), then
Rep(E) = {call,LINE,Rep(E_0),[Rep(E_1), ..., Rep(E_k)]}.
-
If E is E_m:E_0(E_1, ..., E_k), then
Rep(E) =
{call,LINE,{remote,LINE,Rep(E_m),Rep(E_0)},[Rep(E_1), ..., Rep(E_k)]}.
-
If E is a list comprehension [E_0 || W_1, ..., W_k],
where each W_i is a generator or a filter, then
Rep(E) = {lc,LINE,Rep(E_0),[Rep(W_1), ..., Rep(W_k)]}. For Rep(W), see
below.
-
If E is a binary comprehension <<E_0 || W_1, ..., W_k>>,
where each W_i is a generator or a filter, then
Rep(E) = {bc,LINE,Rep(E_0),[Rep(W_1), ..., Rep(W_k)]}. For Rep(W), see
below.
-
If E is begin B end, where B is a body, then
Rep(E) = {block,LINE,Rep(B)}.
-
If E is if Ic_1 ; ... ; Ic_k end,
where each Ic_i is an if clause then
Rep(E) =
{'if',LINE,[Rep(Ic_1), ..., Rep(Ic_k)]}.
-
If E is case E_0 of Cc_1 ; ... ; Cc_k end,
where E_0 is an expression and each Cc_i is a
case clause then
Rep(E) =
{'case',LINE,Rep(E_0),[Rep(Cc_1), ..., Rep(Cc_k)]}.
-
If E is try B catch Tc_1 ; ... ; Tc_k end,
where B is a body and each Tc_i is a catch clause then
Rep(E) =
{'try',LINE,Rep(B),[],[Rep(Tc_1), ..., Rep(Tc_k)],[]}.
-
If E is try B of Cc_1 ; ... ; Cc_k catch Tc_1 ; ... ; Tc_n end,
where B is a body,
each Cc_i is a case clause and
each Tc_j is a catch clause then
Rep(E) =
{'try',LINE,Rep(B),[Rep(Cc_1), ..., Rep(Cc_k)],[Rep(Tc_1), ..., Rep(Tc_n)],[]}.
-
If E is try B after A end,
where B and A are bodies then
Rep(E) =
{'try',LINE,Rep(B),[],[],Rep(A)}.
-
If E is try B of Cc_1 ; ... ; Cc_k after A end,
where B and A are a bodies and
each Cc_i is a case clause then
Rep(E) =
{'try',LINE,Rep(B),[Rep(Cc_1), ..., Rep(Cc_k)],[],Rep(A)}.
-
If E is try B catch Tc_1 ; ... ; Tc_k after A end,
where B and A are bodies and
each Tc_i is a catch clause then
Rep(E) =
{'try',LINE,Rep(B),[],[Rep(Tc_1), ..., Rep(Tc_k)],Rep(A)}.
-
If E is try B of Cc_1 ; ... ; Cc_k catch Tc_1 ; ... ; Tc_n after A end,
where B and A are a bodies,
each Cc_i is a case clause and
each Tc_j is a catch clause then
Rep(E) =
{'try',LINE,Rep(B),[Rep(Cc_1), ..., Rep(Cc_k)],[Rep(Tc_1), ..., Rep(Tc_n)],Rep(A)}.
-
If E is receive Cc_1 ; ... ; Cc_k end,
where each Cc_i is a case clause then
Rep(E) =
{'receive',LINE,[Rep(Cc_1), ..., Rep(Cc_k)]}.
-
If E is receive Cc_1 ; ... ; Cc_k after E_0 -> B_t end,
where each Cc_i is a case clause,
E_0 is an expression and B_t is a body, then
Rep(E) =
{'receive',LINE,[Rep(Cc_1), ..., Rep(Cc_k)],Rep(E_0),Rep(B_t)}.
-
If E is fun Name / Arity, then
Rep(E) = {'fun',LINE,{function,Name,Arity}}.
-
If E is fun Module:Name/Arity, then
Rep(E) = {'fun',LINE,{function,Module,Name,Arity}}.
-
If E is fun Fc_1 ; ... ; Fc_k end
where each Fc_i is a function clause then Rep(E) =
{'fun',LINE,{clauses,[Rep(Fc_1), ..., Rep(Fc_k)]}}.
-
If E is query [E_0 || W_1, ..., W_k] end,
where each W_i is a generator or a filter, then
Rep(E) = {'query',LINE,{lc,LINE,Rep(E_0),[Rep(W_1), ..., Rep(W_k)]}}.
For Rep(W), see below.
-
If E is E_0.Field, a Mnesia record access
inside a query, then
Rep(E) = {record_field,LINE,Rep(E_0),Rep(Field)}.
-
If E is ( E_0 ), then
Rep(E) = Rep(E_0),
i.e., parenthesized expressions cannot be distinguished from their bodies.
4.4.1 Generators and filters
When W is a generator or a filter (in the body of a list or binary comprehension), then:
-
If W is a generator P <- E, where P is a pattern and E
is an expression, then
Rep(W) = {generate,LINE,Rep(P),Rep(E)}.
-
If W is a generator P <= E, where P is a pattern and E
is an expression, then
Rep(W) = {b_generate,LINE,Rep(P),Rep(E)}.
-
If W is a filter E, which is an expression, then
Rep(W) = Rep(E).
4.4.2 Binary element type specifiers
A type specifier list TSL for a binary element is a sequence of type
specifiers TS_1 - ... - TS_k.
Rep(TSL) = [Rep(TS_1), ..., Rep(TS_k)].
When TS is a type specifier for a binary element, then:
-
If TS is an atom A, Rep(TS) = A.
-
If TS is a couple A:Value where A is an atom and Value
is an integer, Rep(TS) = {A, Value}.
4.5 Clauses
There are function clauses, if clauses, case clauses
and catch clauses.
A clause C is one of the following alternatives:
-
If C is a function clause ( Ps ) -> B
where Ps is a pattern sequence and B is a body, then
Rep(C) = {clause,LINE,Rep(Ps),[],Rep(B)}.
-
If C is a function clause ( Ps ) when Gs -> B
where Ps is a pattern sequence,
Gs is a guard sequence and B is a body, then
Rep(C) = {clause,LINE,Rep(Ps),Rep(Gs),Rep(B)}.
-
If C is an if clause Gs -> B
where Gs is a guard sequence and B is a body, then
Rep(C) = {clause,LINE,[],Rep(Gs),Rep(B)}.
-
If C is a case clause P -> B
where P is a pattern and B is a body, then
Rep(C) = {clause,LINE,[Rep(P)],[],Rep(B)}.
-
If C is a case clause P when Gs -> B
where P is a pattern,
Gs is a guard sequence and B is a body, then
Rep(C) = {clause,LINE,[Rep(P)],Rep(Gs),Rep(B)}.
-
If C is a catch clause P -> B
where P is a pattern and B is a body, then
Rep(C) = {clause,LINE,[Rep({throw,P,_})],[],Rep(B)}.
-
If C is a catch clause X : P -> B
where X is an atomic literal or a variable pattern,
P is a pattern and B is a body, then
Rep(C) = {clause,LINE,[Rep({X,P,_})],[],Rep(B)}.
-
If C is a catch clause P when Gs -> B
where P is a pattern, Gs is a guard sequence
and B is a body, then
Rep(C) = {clause,LINE,[Rep({throw,P,_})],Rep(Gs),Rep(B)}.
-
If C is a catch clause X : P when Gs -> B
where X is an atomic literal or a variable pattern,
P is a pattern, Gs is a guard sequence
and B is a body, then
Rep(C) = {clause,LINE,[Rep({X,P,_})],Rep(Gs),Rep(B)}.
4.6 Guards
A guard sequence Gs is a sequence of guards G_1; ...; G_k, and
Rep(Gs) = [Rep(G_1), ..., Rep(G_k)]. If the guard sequence is
empty, Rep(Gs) = [].
A guard G is a nonempty sequence of guard tests Gt_1, ..., Gt_k, and
Rep(G) = [Rep(Gt_1), ..., Rep(Gt_k)].
A guard test Gt is one of the following alternatives:
-
If Gt is an atomic literal L, then Rep(Gt) = Rep(L).
-
If Gt is a variable pattern V, then
Rep(Gt) = {var,LINE,A},
where A is an atom with a printname consisting of the same characters as
V.
-
If Gt is a tuple skeleton {Gt_1, ..., Gt_k}, then
Rep(Gt) = {tuple,LINE,[Rep(Gt_1), ..., Rep(Gt_k)]}.
-
If Gt is [], then
Rep(Gt) = {nil,LINE}.
-
If Gt is a cons skeleton [Gt_h | Gt_t], then
Rep(Gt) = {cons,LINE,Rep(Gt_h),Rep(Gt_t)}.
-
If Gt is a binary constructor <<Gt_1:Size_1/TSL_1, ..., Gt_k:Size_k/TSL_k>>, then
Rep(Gt) = {bin,LINE,[{bin_element,LINE,Rep(Gt_1),Rep(Size_1),Rep(TSL_1)}, ..., {bin_element,LINE,Rep(Gt_k),Rep(Size_k),Rep(TSL_k)}]}.
For Rep(TSL), see above.
An omitted Size is represented by default. An omitted TSL
(type specifier list) is represented by default.
-
If Gt is Gt_1 Op Gt_2, where Op
is a binary operator, then Rep(Gt) = {op,LINE,Op,Rep(Gt_1),Rep(Gt_2)}.
-
If Gt is Op Gt_0, where Op is a unary operator, then
Rep(Gt) = {op,LINE,Op,Rep(Gt_0)}.
-
If Gt is #Name{Field_1=Gt_1, ..., Field_k=Gt_k}, then
Rep(E) =
{record,LINE,Name, [{record_field,LINE,Rep(Field_1),Rep(Gt_1)}, ..., {record_field,LINE,Rep(Field_k),Rep(Gt_k)}]}.
-
If Gt is #Name.Field, then
Rep(Gt) = {record_index,LINE,Name,Rep(Field)}.
-
If Gt is Gt_0#Name.Field, then
Rep(Gt) = {record_field,LINE,Rep(Gt_0),Name,Rep(Field)}.
-
If Gt is A(Gt_1, ..., Gt_k), where A is an atom, then
Rep(Gt) = {call,LINE,Rep(A),[Rep(Gt_1), ..., Rep(Gt_k)]}.
-
If Gt is A_m:A(Gt_1, ..., Gt_k), where A_m is
the atom erlang and A is an atom or an operator, then
Rep(Gt) = {call,LINE,{remote,LINE,Rep(A_m),Rep(A)},[Rep(Gt_1), ..., Rep(Gt_k)]}.
-
If Gt is {A_m,A}(Gt_1, ..., Gt_k), where A_m is
the atom erlang and A is an atom or an operator, then
Rep(Gt) = {call,LINE,Rep({A_m,A}),[Rep(Gt_1), ..., Rep(Gt_k)]}.
-
If Gt is ( Gt_0 ), then
Rep(Gt) = Rep(Gt_0),
i.e., parenthesized guard tests cannot be distinguished from their bodies.
Note that every guard test has the same source form as some expression,
and is represented the same way as the corresponding expression.
4.7 The abstract format after preprocessing
The compilation option debug_info can be given to the
compiler to have the abstract code stored in
the abstract_code chunk in the BEAM file
(for debugging purposes).
In OTP R9C and later, the abstract_code chunk will
contain
{raw_abstract_v1,AbstractCode}
where AbstractCode is the abstract code as described
in this document.
In releases of OTP prior to R9C, the abstract code after some more
processing was stored in the BEAM file. The first element of the
tuple would be either abstract_v1 (R7B) or abstract_v2
(R8B).
erts 5.7.2
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