Call a function: Difference between revisions

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By default, arguments are passed readonly, which allows the implementation to decide whether pass-by-reference or pass-by-value is more efficient on a case-by-case basis. Explicit lvalue, reference, or copy semantics may be requested on a parameter-by-parameter basis, and the entire argument list may be processed raw if that level of control is needed.

=={{header|Phix}}==

Phix has three kinds of routines: procedure, function, and type. A procedure does not return a value, whereas a function does.

A type is a specialised kind of function, that permits declarations of instances of that type, which are

automatically validated whenever they are changed, and further the type routine always returns true or false.

* Phix does not allow implicit discard of function results. The explicit discard statement takes the form

<lang Phix>{} = myfunction()</lang>

* This is in fact a simple contraction of standard multiple assigment (which can be nested):

<lang Phix>{cities,populations} = columize(muncipalities)

{{},populations} = columize(muncipalities) -- discard result[1]

{cities,{}} = columize(muncipalities) -- discard result[2]

{cities} = columize(muncipalities) -- ""</lang>

* Calling a function with no parameters still requires the empty argument list.

* Optional arguments are denoted by the presence of a default, and must be grouped on the right:

<lang Phix>function myfunction(integer a, string b="default")

return {a,b}

end function

--? myfunction() -- illegal, compile-time error

?myfunction(1) -- displays {1,"default"}

?myfunction(2,"that") -- displays {2,"that"}</lang>

* Sequence parameters can be of any length, which is another way to implement optional/variable number of arguments.

* Named arguments can be specified in any order, with an error if any non-optional parameters are missing:

<lang Phix>?myfunction(b:="then",a:=3) -- displays {3,"then"}

--?myfunction(b:="though") -- compile-time error</lang>

* The programmer is free to use either positional parameters or named parameters, or a mixture of both (with positional parameters first).

* Phix does not support first-class functions, but instead uses an integer routine_id mechanism (and obviously integers are first class):

<lang Phix>constant integer r_my_func = routine_id("myroutine")

?call_func(r_my_func,{1}) -- displays {1,"default"}</lang>

The value of r_my_func can be passed as an argument to any routine, or stored in a table, and invoked in a similar fashion.<br>

Note however that for performance reasons some builtins do not have a proper routine_id; if you need one you must write a trivial one-line wrapper.<br>

(For a full list, see psym.e/syminit() calls to AutoAsm(), whereas calls to initialAutoEntry() therein indicate builtins that can have routine_ids.)<br>

(One day the compiler may be enhanced to automatically create one-line wrappers as needed, but that is quite near the end of a fairly long to-do list.)

* Partial application is usually achieved through a single variable-length "user_data" parameter within a call_func() expression.

* All arguments are passed by reference with copy-on-write semantics: to modify the value of a parameter you must both return and assign it, as in:

<lang Phix>s = append(s,item)</lang>

* Implicit forward calls are supported, as are optional explicit forward declarations, which occasionally give better compilation error messages.

=={{header|PicoLisp}}==