The Erlang meta interpreter.

This module provides an interpreter for Erlang expressions. The expressions are in the abstract syntax as returned by erl_parse, the Erlang parser, or io.

Local Function Handler

During evaluation of a function, no calls can be made to local functions. An undefined function error would be generated. However, the optional argument LocalFunctionHandler can be used to define a function that is called when there is a call to a local function. The argument can have the following formats:

• {value,Func} - This defines a local function handler that is called with:

  Func(Name, Arguments)

  Name is the name of the local function (an atom) and Arguments is a list of the evaluated arguments. The function handler returns the value of the local function. In this case, the current bindings cannot be accessed. To signal an error, the function handler calls exit/1 with a suitable exit value.

• {eval,Func} - This defines a local function handler that is called with:

  Func(Name, Arguments, Bindings)

  Name is the name of the local function (an atom), Arguments is a list of the unevaluated arguments, and Bindings are the current variable bindings. The function handler returns:

  {value,Value,NewBindings}

  Value is the value of the local function and NewBindings are the updated variable bindings. In this case, the function handler must itself evaluate all the function arguments and manage the bindings. To signal an error, the function handler calls exit/1 with a suitable exit value.

• none - There is no local function handler.

Non-Local Function Handler

The optional argument NonLocalFunctionHandler can be used to define a function that is called in the following cases:

• A functional object (fun) is called.
• A built-in function is called.
• A function is called using the M:F syntax, where M and F are atoms or expressions.
• An operator Op/A is called (this is handled as a call to function erlang:Op/A).

Exceptions are calls to erlang:apply/2,3; neither of the function handlers are called for such calls. The argument can have the following formats:

• {value,Func} - This defines a non-local function handler. The function may be called with two arguments:

  Func(FuncSpec, Arguments)

  or three arguments:

  Func(Anno, FuncSpec, Arguments)

  Anno is the erl_anno:anno() of the node, FuncSpec is the name of the function on the form {Module,Function} or a fun, and Arguments is a list of the evaluated arguments. The function handler returns the value of the function. To signal an error, the function handler calls exit/1 with a suitable exit value.

• none - There is no non-local function handler.

Note

For calls such as erlang:apply(Fun, Args) or erlang:apply(Module, Function, Args), the call of the non-local function handler corresponding to the call to erlang:apply/2,3 itself (Func({erlang, apply}, [Fun, Args]) or Func({erlang, apply}, [Module, Function, Args])) never takes place.

The non-local function handler is however called with the evaluated arguments of the call to erlang:apply/2,3: Func(Fun, Args) or Func({Module, Function}, Args) (assuming that {Module, Function} is not {erlang, apply}).

Calls to functions defined by evaluating fun expressions "fun ... end" are also hidden from non-local function handlers.

The non-local function handler argument is probably not used as frequently as the local function handler argument. A possible use is to call exit/1 on calls to functions that for some reason are not allowed to be called.