Provided by: erlang-manpages_20.2.2+dfsg-1ubuntu2_all 
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.
DATA TYPES
bindings() = [{name(), value()}]
binding_struct() = orddict:orddict()
A binding structure.
expression() = erl_parse:abstract_expr()
expressions() = [erl_parse:abstract_expr()]
As returned by erl_parse:parse_exprs/1 or io:parse_erl_exprs/2.
expression_list() = [expression()]
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())
non_local_function_handler() = {value, nlfun_handler()} | none
Further described in section Non-Local Function Handler in this module.
value() = term()
EXPORTS
add_binding(Name, Value, BindingStruct) -> binding_struct()
Types:
Name = name()
Value = value()
BindingStruct = binding_struct()
Adds binding Name=Value to BindingStruct. Returns an updated binding structure.
binding(Name, BindingStruct) -> {value, value()} | unbound
Types:
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()
Types:
Name = name()
BindingStruct = binding_struct()
Removes the binding of Name in BindingStruct. Returns an updated binding structure.
expr(Expression, Bindings) -> {value, Value, NewBindings}
expr(Expression, Bindings, LocalFunctionHandler) ->
{value, Value, NewBindings}
expr(Expression,
Bindings,
LocalFunctionHandler,
NonLocalFunctionHandler) ->
{value, Value, NewBindings}
expr(Expression,
Bindings,
LocalFunctionHandler,
NonLocalFunctionHandler,
ReturnFormat) ->
{value, Value, NewBindings} | Value
Types:
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) -> {ValueList, NewBindings}
expr_list(ExpressionList, Bindings, LocalFunctionHandler) ->
{ValueList, NewBindings}
expr_list(ExpressionList,
Bindings,
LocalFunctionHandler,
NonLocalFunctionHandler) ->
{ValueList, NewBindings}
Types:
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) -> {value, Value, NewBindings}
exprs(Expressions, Bindings, LocalFunctionHandler) ->
{value, Value, NewBindings}
exprs(Expressions,
Bindings,
LocalFunctionHandler,
NonLocalFunctionHandler) ->
{value, Value, NewBindings}
Types:
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.
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 that 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 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.
KNOWN LIMITATION
Undocumented functions in this module are not to be used.