# erl_eval

### MODULE

erl_eval

### MODULE SUMMARY

The Erlang Meta Interpreter

### DESCRIPTION

This module provides an interpreter for Erlang expressions. The expressions are in the abstract syntax as returned by `erl_parse`, the Erlang parser, or a call to `io:parse_erl_exprs/2`.

### EXPORTS

`exprs(Expressions, Bindings) -> {value, Value, NewBindings}`
`exprs(Expressions, Bindings, LocalFunctionHandler) -> {value, Value, NewBindings}`
`exprs(Expressions, Bindings, LocalFunctionHandler, NonlocalFunctionHandler) -> {value, Value, NewBindings}`

Types:

`Expressions = as returned by erl_parse or io:parse_erl_exprs/2`
`Bindings = as returned by bindings/1`
`LocalFunctionHandler = {value, Func} | {eval, Func} | none`
`NonlocalFunctionHandler = {value, Func} | none`

Evaluates `Expressions` with the set of bindings `Bindings`, where `Expressions` is a sequence of expressions (in abstract syntax) of a type which may be returned by `io:parse_erl_exprs/2`. See below for an explanation of how and when to use the arguments `LocalFunctionHandler` and `NonlocalFunctionHandler`.

Returns `{value, Value, NewBindings}`

`expr(Expression, Bindings) -> { value, Value, NewBindings }`
`expr(Expression, Bindings, LocalFunctionHandler) -> { value, Value, NewBindings }`
`expr(Expression, Bindings, LocalFunctionHandler, NonlocalFunctionHandler) -> { value, Value, NewBindings }`

Types:

`Expression = as returned by io:parse_erl_form/2, for example`
`Bindings = as returned by bindings/1`
`LocalFunctionHandler = {value, Func} | {eval, Func} | none`
`NonlocalFunctionHandler = {value, Func} | none`

Evaluates `Expression` with the set of bindings `Bindings`. `Expression` is an expression (in abstract syntax) of a type which may be returned by `io:parse_erl_form/2`. See below for an explanation of how and when to use the arguments `LocalFunctionHandler` and `NonlocalFunctionHandler`.

Returns `{value, Value, NewBindings}`.

`expr_list(ExpressionList, Bindings) -> {ValueList, NewBindings}`
`expr_list(ExpressionList, Bindings, LocalFunctionHandler) -> {ValueList, NewBindings}`
`expr_list(ExpressionList, Bindings, LocalFunctionHandler, NonlocalFunctionHandler) -> {ValueList, NewBindings}`

Evaluates a list of expressions in parallel, using the same initial bindings for each expression. Attempts are made to merge the bindings returned from each evaluation. This function is useful in the `LocalFunctionHandler`. See below.

Returns `{ValueList, NewBindings}`.

`new_bindings() -> BindingStruct`

Returns an empty binding structure.

`bindings(BindingStruct) -> Bindings`

Returns the list of bindings contained in the binding structure.

`binding(Name, BindingStruct) -> Binding`

Returns the binding of `Name` in `BindingStruct`.

`add_binding(Name, Value, Bindings) -> BindingStruct`

Adds the binding `Name = Value` to `Bindings`. Returns an updated binding structure.

`del_binding(Name, Bindings) -> BindingStruct`

Removes the binding of `Name` in `Bindings`. Returns an updated binding structure.

### 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` may be used to define a function which 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 which 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, it is not possible to access the current bindings. To signal an error, the function handler just calls `exit/1` with a suitable exit value.
`{eval,Func}`
  This defines a local function handler which 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 just calls `exit/1` with a suitable exit value.
`none`
  There is no local function handler.

### Non-local Function Handler

The optional argument `NonlocalFunctionHandler` may be used to define a function which 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. Exceptions are function calls in guard tests and calls to `erlang:apply/2,3`; neither of the function handlers will be called for such calls. The argument can have the following formats:

`{value,Func}`
  This defines an nonlocal function handler which is called with:

      Func(FuncSpec, Arguments)

  `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 just calls `exit/1` with a suitable exit value.
`none`
  There is no nonlocal function handler.

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])`--will never take place. The non-local function handler will however be 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}`).

The nonlocal 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.

### Bugs

The evaluator is not complete. `receive` cannot be handled properly.

Any undocumented functions in `erl_eval` should not be used.

### AUTHORS

Robert Virding - support@erlang.ericsson.se

stdlib 1.14.1
Copyright © 1991-2006 Ericsson AB