blackcoffee

CoffeeScript + hygienic macros

npm install blackcoffee
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What's this?

BlackCoffee is a CoffeeScript dialect that adds hygienic macros (also known as black magic, hence the name). We aim for inclusion of this exension into mainline CoffeeScript.

Applications include:

  • compile-time calculations,
  • file inclusion,
  • conditional compilation,
  • i18n translation,
  • html resource versioning,
  • syntactic sugar,
  • and whatever else you can think of doing at compile-time.

Besides macros, BlackCoffee sneaks one additional feature in CoffeeScript: Coco-like !-> and !=> procedures. Prefixing an exclamation mark to the function operator, causes the function not to do an implicit return of the last expression.

Installation and usage

You can obtain BlackCoffee through npm:

npm install blackcoffee

..or clone the repository:

git clone https://github.com/paiq/blackcoffee

In both cases you'll have a local installation, assuming you don't want to replace your regular CoffeeScript compiler. You'll find the main compiler executable here blackcoffee/bin/coffee or here node\_modules/blackcoffee/bin/coffee.

The compiler can be used in the exact same way as the regular CoffeeScript compiler (except that macros are supported, of course). In addition, BlackCoffee comes with its own very simple CLI named blackcoffee, which supports multiple source files and command line flags that can be used by macros. Read the 'blackcoffee cli' section for more info.

To install blackcoffee system-wide, replacing any already installed CoffeeScript version:

sudo npm install -g blackcoffee

...or:

git clone https://github.com/paiq/blackcoffee
cd blackcoffee
sudo bin/cake install

One-time macro calls

The macro keyword can be used in two distinct ways. When it's followed directly by a closure, that closure will be executed at compile time. In case the closure returns a BlackCoffee node (see below) it will replace the original macro-call. Otherwise the macro-call will be removed or replaced by undefined.

The compile-time closure calls do not get any arguments. They do get a this-object that will be the same for all calls to macro closures, which can safely be used to maintain state. This object is also reachable through cfg in the global compile-time namespace.

Examples:

macro ->
    @someStateVar = []
# compiles to nothing, because `[]` is not a BlackCoffee node.

macro ->
    macro.fileToNode "test.coffee"
# `macro.fileToNode` reads a js or coffee file, and returns the BlackCoffee
# node for it. As the macro closure returns a node, it will replace the call.
# So in effect, the file is included.

fileContents = macro ->
    macro.valToNode ''+macro.require('fs').readFileSync 'file.txt'
# `macro.valToNode` converts a json-able value to a BlackCoffee node. So
# `fileContents` will contain whatever is in 'file.txt'. Of course, as we're
# using `require('fs')` this will not work when compiling in the browser.

Named macro definitions

When the macro keyword is followed by an identifier and then a closure, it's a definition. Every time the identifier is used as a closure call later on in the program, the closure call is replaced by the result of a compile-time call to the defined macro. Arguments to the macro are passed to the compile-time closure as BlackCoffee nodes.

Examples:

macro macroWithoutArgs -> macro.csToNode '3 * a'
# A named macro is defined here, but no code is generated yet.

z = macroWithoutArgs() + 4
# Here we're actually using the macro. Expands to: `z = 3 * a + 4`.

macro replaceFooWithBar (node) -> node.subst {foo: macro.csToNode 'bar'}
# Here, the macro takes an argument. `subst` is a method that can be used on
# every type of BlackCoffee node, to recursively search and replace
# identifiers with a node.

arr = replaceFooWithBar -> [foo+foo, -> foo]
# Expands to: `arr = [bar+bar, -> bar]`

Helper functions

We've already seen some examples of helper functions defined in the compile-time macro. namespace. Let's go over them one by one:

macro.valToNode(value) Convert a json-able value to a node.

buildInfo = macro -> macro.valToNode
    time: new Date().getTime()
    host: macro.require('os').hostname()

macro.csToNode(scriptString) Parse a string of BlackCoffee script and return its main node.

macro -> macro.csToNode "x = a+b+(#{process.env.buildMagic})+4"

macro.jsToNode(scriptString) Creates a literal node, embedding a piece of unprocessed Javascript. As this node just embeds an opaque piece of Javascript, subst (see below) will not have any effect on it.

macro -> macro.jsToNode @someFancyToolThatGeneratesJavascript()

macro.fileToNode(filename,language) A little wrapper around csToNode and jsToNode that tries to load filename from disk using fs.readFileSync. Of course, this won't work when compiling in the browser. If language is 'js' or 'cs', that is how the file will be interpreted. If it is not set, fileToNode will make a guess based on the file extension.

This helper can be used for file inclusion, though one may want to write a wrapper macro around it, to search in the appropriate path(s) and maybe to dependency tracking and such.

macro -> macro.fileToNode 'includeMe.coffee'
# or
macro include (fn) -> macro.fileToNode @myPathSearcher(fn+'.coffee')
include 'includeMe'

macro.codeToNode(closure) Takes a closure (function), and returns its body as a node. So the closure itself is not part of the node. Note that codeToNode is the only helper function that is not actually a function. It's a predefined macro. It needs to be, because it needs to capture the node of its argument instead of the value.

macro swap (a,b) -> macro.codeToNode(-> [x,y]=[y,x]).subst {x:a, y:b}
swap c, d

macro.nodeToVal(node) Try to evaluate the node at compile-time and return the value. Any errors (such as referencing run-time variables), will throw at compile-time.

# Example for those who long for the bad old C-preprocessor days. ;-)
macro ifdef (key) ->
    key = macro.nodeToVal key
    macro.csToNode(if @defines[key] then "if true" else "if false")
throw 'party' ifdef 'firstBeta'

macro.idToVal(node) In case node is a bare identifier, return its string value. Return undefined otherwise.

# Example for those who long for the bad old C-preprocessor days. ;-)
macro ifdef (key) ->
    key = macro.nodeToId key
    macro.csToNode(if @defines[key] then "if true" else "if false")
throw 'party' ifdef firstBeta

Working with Nodes

WARNING: Manipulating BlackCoffee nodes directly exposes compiler implementation details. Therefore, documentation is mostly absent and things may change between compiler versions without notice. It is recommended to use these techniques as little as possible.

macro.[NodeType] The compile-time macro object provides direct access to BlackCoffee node classes. You can use these to test if an argument node has some specific type, or to programatically generate nodes. Node types and their constructor arguments can be found in the compiler sources, in nodes.coffee.

macro something (arg) ->
    if arg instanceof macro.Bool
        @someting()
    else
        @somethingElse()
macro debug (args...) ->
    if @debugging
        new macro.Call(new macro.Literal("debugImpl"), args)

macro.walk(node,visitor) For each of nodes (recursive) child nodes, call visitor(child). In case the visitor returns a BlackCoffee node, that node is used to replace the original child. If false is returned, the child is removed (or replaced by undefined if it cannot be removed). Otherwise, the original child is left unmodified.

macro delegateArithmetic (func) ->
    operators =
        '+': 'add'
        '-': 'sub'
        '*': 'mult'
    macro.walk func.body, (node) ->
        if node instanceof macro.Op and (op = operators[node.operator])
            macro.csToNode("(a).#{op}(b)").subst {a: node.first, b: node.second}

# Next, we'd have to define the `add/sub/mult` methods for the types that we'll
# be working with in a `delegateArithmetic` section.
Array::add = (other) -> 42 # do something smart here instead
Number::sub = (other) -> -42
# etc...

# So, now this uses regular javascript operators:
eq 7, 3+4
eq "3,45,6", [3,4]+[5,6]

delegateArithmetic ->
    # And this uses the arithmetic methods defined on the prototypes:
    eq [3,4,5], [1,2,3]+2  
    eq 20, [1,2,3]*[2,3,4]
    eq [1,0,1], [3,2,4]-[2,0,3]
    eq 9, 3*3

blackcoffee cli

This version of CoffeeScript comes with the usual bin/coffee compiler/runner/repl, adapted to work with macros. You may choose to use the additional bin/blackcoffee compiler instead, as it allows for compilation of multiple source files to one target. This is especially useful if you want to prefix each script with a set of common macro definitions.

Syntax: bin/blackcoffee [-o OUTFILE_JS] [-m OUTFILE_SRCMAP] [-f KEY[=VAL]]... [--] [INFILE]...

I think these options are pretty much self evident, except for -f, which can be used to set key/values on the flags object available in the compile-time context. This allows you to specify any sort of build options in your build script, which you can then use in your macros.

Example:

bin/blackcoffee -o test.js -m test.srcMap -f dev macros.coffee test.coffee

macros.coffee:

macro LOG (str) ->
    macro.codeToNode(->console.log x).subst {x: str} if flags.dev

test.coffee:

# We can use macros defined in the other file here. The following will
# compile to nothing, unless the '-f dev' flag is specified at compile time:
LOG "Hello developers!"
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