Directives - language feature

Example from JavaScript, TypeScript:

"use strict";
"use asm";

Example from Python, Cython:

from __future__ import feature
# coding=<encoding name>

Example from C, C++, Objective-C, Tick C:

#include <stdio.h>
#define height 10
#ifdef
#endif
#if
#else
#ifndef
#undef
#pragma

Example from Perl:

# In Perl, the keyword "use", which imports modules, can also be used to specify directives, such as use strict; or use utf8;
use utf8;

Example from Ruby:

coding: UTF-8

Example from C#:

#define MAX_CLIENTS 200
int array[MAX_CLIENTS];

#if PRODUCTION
//code
#elif DEVELOPMENT
//code
#else
//code
#endif

Example from Rust:

// A conditionally-compiled module
#[cfg(target_os = "linux")]
mod bar {
    /* ... */
}
// General metadata applied to the enclosing module or crate.
#![crate_type = "lib"]
// A function marked as a unit test
#[test]
fn test_foo() {
    /* ... */
}
// A lint attribute used to suppress a warning/error
#[allow(non_camel_case_types)]
type int8_t = i8;

// Inner attribute applies to the entire function.
fn some_unused_variables() {
  #![allow(unused_variables)]

  let x = ();
  let y = ();
  let z = ();
}

Example from Haskell:

{-# INLINE foo #-}

Example from Visual Basic:

Option Explicit On|Off
Option Compare Binary

Example from Erlang:

-define(TIMEOUT, 200).
...
call(Request) ->
   server:call(refserver, Request, ?TIMEOUT).
-undef(Macro).

Example from F#:

#if VERSION1
let function1 x y =
   printfn "x: %d y: %d" x y
   x + 2 * y
#else
let function1 x y =
   printfn "x: %d y: %d" x y
   x - 2*y
#endif
// Line directives as source maps can be used when compiling to F#:
#line 25 "C:\\Projects\\MyProject\\MyProject\\Script1"

Example from ALGOL 68, Mary:

.PR POINT .PR
.PR UPPER .PR
.PR RES .PR
'pr' quote 'pr'

Example from FML:

# Directives are special lines that have syntax and semantics of their own. Directives all start with a name of the form .foo; new directives may be added by future versions of the language.
.import math.constants
area = x * math.constants.Pi, ^ 2
.from math.constants import Pi, E
area = x sq, * Pi
polar = a * [E ^ [b * 0+j1]]

Languages with Directives include JavaScript, Python, C, C++, Perl, Ruby, C#, Ada, Rust, Haskell, TypeScript, Objective-C, Visual Basic, Erlang, F#, Common Lisp, Cython, ALGOL 68, Jule, C2, Mary, FML, ScriptEase, Tick C

Languages without Directives include C3, progsbase

This question asks: Is the language embedded in a tiny compile-time directive language?

Read more about Directives on the web: 1.

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