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PureScript (/ˈpjʔr.skrʔpt/^[1]) is a Functional programming language which compiles to JavaScript. JavaScript does not support powerful abstractions, as a result it cannot utilize full capability of Functional Programming. PureScript addresses this issue by allowing expressive, readable code.

PureScript is a statically typed language. Unlike in other dynamically typed languages, in PureScript, types exist only at compile time, and have no representation at runtime. Programs which are correct will be given a type by the compiler, which illustrates its behavior. Conversely, programs which are not given any type are incorrect.

Getting started

Firstly, Setup a working environment for PureScript development. Following are the tools required:

psc - PureScript Compiler
npm -Node package Manager
Pulp - Command line tool

Download latest version of PureScript binary from Github ^[2] and setup the environment variables. Try executing the below command in command line to test:

$psc

Following command is to install the Pulp command line tool:

$ npm install -g pulp

Now you have all tools required to write first PureScript library.

Simple Hello World!

By using pulp init command , create project in an empty directory

$ mkdir my_pscript
$ cd my_pscript
$ pulp init
* Generating project skeleton in ~/my-pscript
$ ls
  bower.json	src		test

Two directories src ,test and a configuration file bower.json are created. src directory contains source files and test directory contains tests performed.

Now Modify src/Main.purs file to create Hello World Program.

module Main where
import Control.Monad.Eff.Console
main = log "Hello, World!"

Now let's build and run the above code.

$ pulp run

* Building project in ~/my-pscript
* Build successful.
Hello, World!

Features

PureScript is a strong and statically typed compile-to-JS language with a number of interesting features, such as:

Types and Type Inference

Types in PureScript are inspired from ML and Haskell and are not similar to the types normally seen in languages like Java or C#, although they serve the same purpose at a high level. Type system in purescript supports type inference . Following is a simple example code which defines a number, but there is no reference to the Number type anywhere in the code. ^[3]

iAmANumber = 
  let square x = x * x
  in square 42.0

Pattern matching

pattern matching is a technique that allows to code compact functions which express complex ideas. Algebraic data types is a similar feature and is closely related to pattern matching. Below is a Simple Pattern Matching function that calculates GCD of two numbers using Euclidean Algorithm.

gcd :: Int -> Int -> Int
gcd n 0 = n
gcd 0 m = m
gcd n m = if n > m then gcd (n - m) m else gcd n (m - n)

Pattern matching allows to code simple and declarative cases. Functions work by pairing sets with results. Each line in the function is a case, expressions on left of equals sign are patterns. Each case is evaluated and the first case which matches is returned.

There are various types of patterns like Integer literal, Variable, Wildcard and Boolean literal patterns.

Simple FFI

FFI(Foriegn Function Interface) is a feature provided by PureScript that enables communication between PureScript and JavaScript code. FFI is very flexible and it allows developer to choose between using foreign functions or type system. JavaScript functions might return not null values which have to be handled by the developer when designing these interfaces.

Call the above PureScript module from javascript, using FFI.

var Test = require('Test');
Test.gcd(15)(20);

Modules

Module keyword is used to refer to modules and all the code in PureScript is written in a module.

module A where
id x = x

When a module is imported(using import keyword) aliases are created for all values and types in it. Values, type constructors and data structures can be explicitly imported. Below is an example to import type constructors by passing a list of data constructors in parentheses.

module B where
import A (runFoo, Foo(..), Bar(Bar))

Modules can be exported by providing a set of names in parenthesis in the declaration similar to import.

Type Classes

Type classes is a powerful kind of abstraction provided by PureScript's type system. Following is the example of show, which is defined by type class in the Prelude module called Show.

class Show a where
  show :: a -> String

Below are some some standard type classes defined in the Prelude and standard libraries.

Eq: Eq type class is used to test two values for equality.

class Eq a where

 eq :: a -> a -> Boolean

Ord: The Ord type class is used to compare two values for types which support ordering.

data Ordering = LT | EQ | GT

class (Eq a) <= Ord a where

 compare :: a -> a -> Ordering

Num: The Num type class is used to identify the types which support addition, subtraction, multiplication and division operations. class (DivisionRing a) <= Num a

DO Notation

Do notation is a special syntax which is used to improve the readability of the code. It uses the keyword Do to introduce a block of code which uses do notation. It makes the structure of algorithm clearer. Following is the factors function written using do notation.

factors :: Int -> Array (Array Int) factors n = filter (\xs -> product xs == n) $ do

 i <- 1 .. n
 j <- i .. n
 return [i, j]

Syntax

Functions and Records

Functions and records are the building blocks of PureScript.

Simple Types

PureScript defines seven built-in types: numbers, strings, booleans, integers, arrays, records, and functions which are defined in Prim module (implicitly imported in every module). These can be seen in PSCi by using the :type command. Following are a few examples.

$ pulp psci

> :type 1.0
Prim.Number

> :type "test"
Prim.String

> :type true
Prim.Boolean

> :type 1
Prim.Int

Unlike JavaScript, all the elements of PureScript array should be of same type.

> :type [1, 2, 3]
Prim.Array Prim.Int

> :type [true, false]
Prim.Array Prim.Boolean

> :type [1, false]
Cannot unify Prim.Int with Prim.Boolean.

Records are PureScript's objects and the syntax of Record literals is same as that of JavaScript's Object literals.

> let author = { name: "Phil", interests: ["Functional Programming", "JavaScript"] }

> :type author
{ name :: Prim.String, interests :: Prim.Array Prim.String }

Dot is used to access the fields of records.

> author.interests
["Functional Programming","JavaScript"]

Functions

Functions can be defined inline ( using \ followed by space) or they can be defined at the top-level of file (by specifying arguments before equal sign).

> let 
    add :: Int -> Int -> Int
    add = \x y -> x + y

or 

add :: Int -> Int -> Int
add x y = x + y

Version History

Following table is based on the information provided in Github. ^[5]

Version	Release date	Changes	Enhancements	Bug Fixes
Old version, no longer maintained: V0.5.0 Polymorph	Apr 27, 2014	Block support has been removed Naming scheme of prelude modules is similar to haskell	Pretty printing for row types has been improved	Overlapping variables in typeclass instances are rejected A bug in the optimizer related to inlining was fixed
Old version, no longer maintained: V0.5.4	Aug 3, 2014			Provides bug fixes and features required to compile the latest core libraries
Old version, no longer maintained: V0.5.5	Sep 2, 2014
Old version, no longer maintained: V0.5.6	Sep 21, 2014
Old version, no longer maintained: V0.5.6.1	Sep 22, 2014
Old version, no longer maintained: V0.5.6.2	Sep 22, 2014
Old version, no longer maintained: V0.5.6.3	Oct 6, 2014
Old version, no longer maintained: V0.5.7	Oct 29, 2014
Old version, no longer maintained: V0.5.7.1	Oct 30, 2014
Old version, no longer maintained: V0.5.6.0 Holoship	Nov 8, 2014	Alternative type class hierarchy has been refactored Runtime type checks has been removed	Tail call optimization has been improved for functions using case expressions. Generated code for type class dictionaries was simplified	Binding group errors in type class members are now caught at compile time.
Old version, no longer maintained: V0.6.0.2	Nov 9, 2014			Prevent psci and psc-make from rebuilding everything on every build
Old version, no longer maintained: V0.6.0.1	Nov 17, 2014	The body of a guarded expression must now be indented past the guard. The body of a guarded expression must now be indented past the guard.	The body of a guarded expression must now be indented past the guard. Import instances when referencing qualified values
Old version, no longer maintained: V0.6.1.1	Nov 19, 2014	The pipe symbol is now a reserved operator. The operators in the Bits type class have been renamed.	Relax indentation requirements
Old version, no longer maintained: V0.6.1.2	Nov 24, 2014
Old version, no longer maintained: V0.6.2	Nov 28, 2014	Command line options with multiplicity 1 now require an equals symbol	Use optparse-applicative instead of cmdtheline
Old version, no longer maintained: V0.6.3	Jan 8, 2015		There is a new "core functional representation", which will enable certain optimizations, and new features such as rewrite rules.	Case statement at end of Eff block not being executed.
Old version, no longer maintained: V0.6.4	Jan 23, 2015			Fix some precedence issues in the code generator. Fixed a bug in the typechecker.
Old version, no longer maintained: V0.6.4.1	Feb 3, 2015			Various small bug fixes.
Old version, no longer maintained: V0.6.5	Feb 8, 2015		Lightweight record constructors are now supported Syntactic sugar has been introduced for object update functions	Some command line options were fixed in psc-make Some module import errors were fixed
Old version, no longer maintained: V0.6.6	Feb 8, 2015	The syntax of record getters was changed to _.prop	The record part of a record updater can now be made into a wildcard	Newline issues were fixed in executables
Old version, no longer maintained: V0.6.7	Feb 12, 2015
Old version, no longer maintained: V0.5.7.1	Oct 30, 2014
Old version, no longer maintained: V0.5.7.1	Oct 30, 2014

^ "PureScript". Collins English Dictionary – Complete & Unabridged 2012 Digital Edition. William Collins Sons & Co.
^ "PureScript Binary". Github.com.
^ "PureScript – Types". Learnpub.com.
^ "Rapydscript bitbucket repository". Atlassian Bitbucket. 3 Apr 2013. Retrieved 11 May 2014.
^ "PureScript – version history". Github.

[1] "PureScript". Collins English Dictionary – Complete & Unabridged 2012 Digital Edition. William Collins Sons & Co.

[2] "PureScript Binary". Github.com.

[3] "PureScript – Types". Learnpub.com.

[4] "Rapydscript bitbucket repository". Atlassian Bitbucket. 3 Apr 2013. Retrieved 11 May 2014.

[5] "PureScript – version history". Github.

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