purerl - the PureScript Erlang backend

Overview

PureScript is a small strongly typed programming language with expressive types, written in and inspired by Haskell; purerl is a PureScript backend targetting Erlang source.

Resources

• FP slack channel #purerl - general discussion, support, news
• purerl Pursuit - a version of the PureScript pursuit documentation repository for purerl package sets
• The purerl organisation hosts ports of some core libraries.

Versions

Currently the purerl executable should correspond to purs compiler versions as follows:

Usage

The purerl backend requires the mainline PureScript compiler (purs) to operate. It generates .erl source files from the CoreFn purs compiler output (and externs.json type information), which can then be compiled by erlc.

The recommended usage is with spago, which now supports alternative backends. In spago.dhall set

{ name = "my-project",
, backend = "purerl"
, ...
}

and in packages.dhall include a purerl-specific package set, e.g.

let upstream =
      https://raw.githubusercontent.com/purerl/package-sets/erl-0.13.5-20191204/src/packages.dhall sha256:308bc2dd2c5700e6e22fae85b10472fd76ddaaf8034711ff0eaede145573479a

Then

• spago build will build the project through to .erl
• spago run will run the project from a given entry point

Usage without spago

First compile to the CoreFn representation, e.g.

• purs compile 'my_deps/**/*.purs' 'src/**/*.purs' --codegen corefn
• pulp build -- --codegen corefn
• psc-package build -- --codegen corefn

Then run purerl to generate .erl files

• purerl

(No parameters are required, purerl will build whatever is in output/).

IDE support

The standard PureScript IDE tooling based on purs ide and editor plugins and/or PureScript Language Server should work with purerl projects, but should be configured to generate corefn rather than js, eg with vscode:

"purescript.codegenTargets": [ "corefn" ]

purerl erlang representation and FFI usage

Module names Foo.Bar are transformed to a lower-snake cased form foo_bar (any non-initial uppercase chars will be preserved as such), with a suffix @ps to avoid clashing with built-in erlang modules.

Top level declarations are uniformly output as nullary functions. Identifiers are preserved, with quoting if required. Thus a normal invocation of the output will look like ([email protected]:main())().

Types

FFI

In place of .js FFI files, the Erlang backend has .erl FFI files. As per the regular compiler since 0.9, these must be placed along the corresponding .purs file with the same name.

Module name: Foo.MyModule PureScript file Foo/MyModule.purs Erlang file: Foo/MyModule.erl Erlang module: [email protected]

Note that the FFI code for a module must not only be in a file named correctly, but the module must be named the same as the output module with @foreign appended (so not following the Erlang module naming requirement until this gets copied to output).

FFI files MUST export explicitly the exact set of identifiers which will be imported by the corresponding PureScript file. The compiler will check these exports and use them to inform codegen.

Auto-currying: functions can be defined with any arity. According to the arity of the export (parsed from the export list) the compiler will automatically apply to the right number of arguments. By extension, values are exported as a function of arity 0 returning that value.

An example:

module Foo.Bar where

foreign import f :: Int -> Int -> Int -> Int

-module(foo_bar@foreign).
-export([f/3]).

f(X, Y, Z) -> X + Y * Z.

This could also have been defined as

-module(foo_bar@foreign).
-export([f/1]).

f(X) ->
  fun (Y) ->
    fun (Z) ->
      X + Y * Z
    end
  end.