erl_eval(3)

NAME
erl_eval - 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 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:

    • functional object (fun) is called.

    • built-in function is called.

    • 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 of 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.

DATA TYPES

binding_struct() :: orddict:orddict() | map().

    A binding structure. It is either a map or an orddict. erl_eval will always return the same type as the one given.
bindings() :: [{name(), value()}].
expression() :: erl_parse:abstract_expr().
expression_list() :: [expression()].
expressions() :: [erl_parse:abstract_expr()].

    As returned by erl_parse:parse_exprs/1 or io:parse_erl_exprs/2.
func_spec() :: {Module :: module(), Function :: atom()} | function().
lfun_eval_handler() :: fun((Name :: atom(), Arguments :: expression_list(), Bindings :: binding_struct()) -> {value, Value :: value(), NewBindings :: binding_struct()}).
lfun_value_handler() :: fun((Name :: atom(), Arguments :: [term()]) -> Value :: value()).
local_function_handler() :: {value, lfun_value_handler()} | {eval, lfun_eval_handler()} | none.

    Further described in section Local Function Handler in this module
name() :: term().
nlfun_handler() :: fun((FuncSpec :: func_spec(), Arguments :: [term()]) -> term()) | fun((Anno :: erl_anno:anno(), FuncSpec :: func_spec(), Arguments :: [term()]) -> term()).
non_local_function_handler() :: {value, nlfun_handler()} | none.

    Further described in section Non-Local Function Handler in this module.
value() :: term().

FUNCTIONS

add_binding(Name, Value, BindingStruct) -> binding_struct() when Name :: name(), Value :: value(), BindingStruct :: binding_struct().

    Adds binding Name=Value to BindingStruct. Returns an updated binding structure.
binding(Name, BindingStruct) -> {value, value()} | unbound when Name :: name(), BindingStruct :: binding_struct().

    Returns the binding of Name in BindingStruct.
bindings(BindingStruct :: binding_struct()) -> bindings().

    Returns the list of bindings contained in the binding structure.
del_binding(Name, BindingStruct) -> binding_struct() when Name :: name(), BindingStruct :: binding_struct().

    Removes the binding of Name in BindingStruct. Returns an updated binding structure.
expr(Expression, Bindings, LocalFunctionHandler, NonLocalFunctionHandler, ReturnFormat) -> {value, Value, NewBindings} | Value when Expression :: expression(), Bindings :: binding_struct(), LocalFunctionHandler :: local_function_handler(), NonLocalFunctionHandler :: non_local_function_handler(), ReturnFormat :: none | value, Value :: value(), NewBindings :: binding_struct().

    Evaluates Expression with the set of bindings Bindings. Expression is an expression in abstract syntax.

    For an explanation of when and how to use arguments LocalFunctionHandler and NonLocalFunctionHandler, see sections Local Function Handler and Non-Local Function Handler in this module.

    Returns {value, Value, NewBindings} by default. If ReturnFormat is value, only Value is returned.
expr_list(ExpressionList, Bindings, LocalFunctionHandler, NonLocalFunctionHandler) -> {ValueList, NewBindings} when ExpressionList :: expression_list(), Bindings :: binding_struct(), LocalFunctionHandler :: local_function_handler(), NonLocalFunctionHandler :: non_local_function_handler(), ValueList :: [value()], NewBindings :: binding_struct().

    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 LocalFunctionHandler, see section Local Function Handler in this module.

    Returns {ValueList, NewBindings}.
exprs(Expressions, Bindings, LocalFunctionHandler, NonLocalFunctionHandler) -> {value, Value, NewBindings} when Expressions :: expressions(), Bindings :: binding_struct(), LocalFunctionHandler :: local_function_handler(), NonLocalFunctionHandler :: non_local_function_handler(), Value :: value(), NewBindings :: binding_struct().

    Evaluates Expressions with the set of bindings Bindings, where Expressions is a sequence of expressions (in abstract syntax) of a type that can be returned by io:parse_erl_exprs/2.

    For an explanation of when and how to use arguments LocalFunctionHandler and NonLocalFunctionHandler, see sections Local Function Handler and Non-Local Function Handler in this module.

    Returns {value, Value, NewBindings}
new_bindings() -> binding_struct().

    Returns an empty binding structure.