Simple Preprocessor

This Transformer modifies code according to directives similar to those found in C preprocessors. The syntax should be familiar.

Let's start with an example:

_coreAjax = $['core-ajax'];  // Get <core-ajax> element.
/* #if DEBUG */
_coreAjax.url = "mock.json";  // Change actual prod URL to a mock file.
/* #endif */

The URL-changing line will only be included in debug builds (which is also the default mode for pub serve). The line will not exist in release builds.

Since you can change build mode to any custom string by running pub build --mode=<mode>, you can have things like this:

/* #if LOCALHOST */
baseUrl = "http://localhost/";
/* #endif */

How to use this

In your pubspec.yaml, add simple_preprocessor as a dependency and as a Transformer. Like this:

  simple_preprocessor: any
- simple_preprocessor

Try to include it before any other transformer (it's a pre-processor, after all).


This is not a best practice. You're changing semantics of your program by a non-standard control mechanism that — worse still — lives in a comment. The only thing that kind of makes this almost okay is that the #if directive is so well recognized.

Even so, if you choose to use it, you shouldn't probably use it for anything more than a few simple #if DEBUG statements.

Supported directives

Currently, there's only #if, #else and #endif.

The #if directive takes one argument, which must be all upper case. This will be compared to the mode of pub build ("release" by default) or pub serve ("debug" by default). When there's a match, anything between #if and #endif (or #else) will be included. If not, it will be stripped.

The #elif directive is on my TODO, but probably not anything else. This shouldn't be a simplified programming language like that of a C preprocessor. If you need to do something more complex than #if DEBUG, you're better off using Dart.

There is no #define – the only definition is made by the transformer mode.

Commenting out branches

Sometimes, to play better with static analysis tooling, you might want to hide some of the lines from it. Here's a solution:

  /* #if DEBUG *//*
  var speed = 1000;
*//* #else */
  var speed = 100;
  /* #endif */

Without the extra /* and */, your IDE would probably display an error about defining the speed variable twice. But simple_preprocessor correctly identifies this syntax and will uncomment the contents between #if end #else for debug builds.

Note that the extra comment must come immediately after the #if or #else comment and directly before the #else or #endif comment. No whitespace is permitted.