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License: MIT License
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Tested with PureScript version


A fast alternative to Purescript's psc-bundle to be used during development.


One great thing about programming in JavaScript is the speed of development. Just edit your source code file, and immediately reload your browser to instantly see your changes.

With PureScript, you must go through a compilation process. Even if it only takes a few seconds, the lag becomes frustrating when trying to iterate rapidly.

But we can try to bring the compilation time down to almost nothing! This project manages to do so for the psc-bundle stage of compilation. It is a tool called psc-bundle-fast that replaces the official psc-bundle tool that comes with PureScript.

psc-bundle-fast should be used only during development. For production you should still use the official psc-bundle since it does dead code elimination and will produce smaller output files.


So how much faster is it? Results for a sample project:

Command Time Output .js Size
psc-bundle 1.458s 108K
psc-bundle-fast 0.091s 464K

That's 16x faster! It's bigger because it contains lots of library code that is not being used (regular psc-bundle strips this out). But for local development, the larger file size has negligible impact on load time, and no impact on performance.

What about browserify and webpack?

They are even slower than PureScript's psc-bundle. Feel free to run your own benchmarks (and tell us the results!)


You need GHC and cabal.

$ cabal update
$ cabal install purescript-bundle-fast

Example Usage

First, use psc as usual to compile your program:

$ psc     './bower_components/**/src/*.purs' \
    --ffi './bower_components/**/src/*.js' \
          './src/**/*.purs' \
    --ffi './src/**/*.js' \
    -o output

Now, just for a comparison, here is how we'd use the regular psc-bundle:

$ psc-bundle './output/**/*.js' -m Main --main Main -o app.js

And here is how you would use psc-bundle-fast instead of the previous step:

$ psc-bundle-fast -i output -m Main --main Main -o app.js

Differences with psc-bundle and limitations

Unlike psc-bundle, psc-bundle-fast does not use a real JavaScript parser. Therefore:

  1. It is not able to perform dead code elimination the way that psc-bundle does, so it will produce output files that are larger.

  2. It will not detect syntax errors in foreign.js files. (This is actually an advantage since the error messages that psc-bundle generates are confusing. It's more helpful to see the error that the browser shows).

  3. foreign.js files that use require to load external JavaScript modules/libraries will not work. These foreign.js files will load, but if they are executed then an error will be triggered. If you need to a PureScript library that has such require usage, then you will need to externally load the required JavaScript library, and then create a stub function called "require" that hooks into it. (If you succeed to do this then share with us how you did it!)

  4. The custom parser that psc-bundle-fast uses is brittle and relies on the specific format that psc outputs. If psc ever makes (even slight) changes to its output then psc-bundle-fast will break.


psc-bundle-fast - Bundles compiled PureScript modules for the browser (fast
version, for development)

Usage: psc-bundle-fast (-i|--input-dir DIR) [-o|--output FILE]
                       (-m|--module MODULE) [--main MODULE] [-n|--namespace ARG]

Available options:
  --version                Show the version number
  -h,--help                Show this help text
  -i,--input-dir DIR       The directory containing the compiled modules. This
                           directory should contain a subdirectory for each
                           compiled module(with the name of the module), and
                           within each of those there should be an index.js (and
                           optional foreign.js) file. The psc compiler usually
                           calls the desired directory "output"
  -o,--output FILE         The output .js file (Default is stdout)
  -m,--module MODULE       Entry point module name(s). All code which is not a
                           transitive dependency of an entry point module will
                           be removed.
  --main MODULE            Generate code to run the main method in the specified
  -n,--namespace ARG       Specify the namespace that PureScript modules will be
                           exported to when running in the
                           browser. (default: "PS")