PureScript For Haskellers

Some info that supposed to help to understand PureScript from Haskell perspective.

If you already know js it will be even simplier.

About PureScript

PureScript written in Haskell but usually distributed as binaries via NPM.

It uses Bower instead of NPM because Bower have flat dependencies and better dependency resolution.
This explained better here: http://harry.garrood.me/blog/purescript-why-bower/

PureScript is strict by default!

Useful links

• https://pursuit.purescript.org (kinda like https://stackage.org for Haskell)
• http://try.purescript.org (online REPL)

Chatting

• #purescript on Freenode
  Matrix bridge: https://riot.im/app/#/room/#freenode_#purescript:matrix.org
• https://gitter.im/purescript/purescript
  Matrix bridge: https://riot.im/app/#/room/#gitter_purescript=2Fpurescript:matrix.org

From Haskell perspective

1. About Prelude:

   PureScript acts like {-# LANGUAGE NoImplicitPrelude #-} in Haskell, and Prelude also isn't distributed with PureScript compiler.

   You need to install dependency purescript-prelude and to import it:

   import Prelude

2. About forall:

   PureScript acts like {-# LANGUAGE ExplicitForAll #-} in Haskell.

   You need to explicitly declare forall for every polymorphic type variable.

3. About unicode:

   In PureScript using unicode is allowed by default.

   Basic unicode symbols also included (such as ∷, ←, →, ⇐, ⇒, ∀, etc.)

4. Basic operators equivalents (those which differ):

   PureScript	Haskell
   (<<<)	(.)
   (>>>)	flip (.) or (.>) from flow package
   (#)	(Data.Function.&) from base package
   (<#>)	(Data.Functor.<&>) from base package
   (<>) (Semigroup type class)	(++)
   a *> b (Apply type class)	a >> b
   b <* a (Apply type class)	b << a

5. Basic functions equivalents (those which differ):

   PureScript	Haskell
   map (Functor type class)	fmap (works like map for lists)
   unsafeThrow from Control.Monad.Eff.Exception.Unsafe from purescript-exceptions package	error
   forkAff from Control.Monad.Aff from purescript-aff package	forkIO

   If you're looking for Haskell's Control.Concurrent.MVar look at PureScript's Control.Monad.Aff.AVar from purescript-aff package.

6. About point-free style:

   For partially applied operators you must specify ghost place for a value:

   PureScript	Haskell
   (_ + 2)	(+ 2)
   (2 + _)	(2 +)

   You're defenitely familiar with {-# LANGUAGE LambdaCase #-} in Haskell:

   In PureScript you have kinda the same, but again, you need to explicitly set ghost place for a value:

   case _ of
     Just x  -> 34
     Nothing -> 42

   That in Haskell would be:

   \case
     Just x  -> 34
     Nothing -> 42

   To update a record in PureScript you also use a ghost place marker:

   _ { foo = 42 }

   But in PureScript you also able to easily modify nested records without even using stuff like lenses:

   _ { foo { bar { baz = 42 } } }

   You able create a function that fills record values this way:

   { foo: _, bar: _ }

   Which is equivalent to:

   \foo bar -> { foo: foo, bar: bar }

   Or even to (as in js):

   \foo bar -> { foo, bar }

7. About Unit:

   PureScript	Haskell
   Type Unit	Type ()
   Value unit	Value ()

8. About IO:

   If you're looking what would be equivalent to IO () in Haskell or just wondering what the heck is Eff (foo :: FOO) Unit:

   PureScript have improved implementation of IO monad in Haskell, the main difference is that Eff monad (in PureScript) have additional parameter to specify limitation of possible side-effects (such as CONSOLE, DOM, REF, etc.) so you can have more precise control of IO stuff.

   You defenitely should read official docs about this, the story couldn't be told in few sentences.

   Few tips about Eff (Eff means effects):

   • IO () is kinda forall eff. Eff eff Unit;
   • You must type your own Eff monads providing type of side-effects which it going to make (e.g. Eff (console :: CONSOLE) Unit);
   • But usually it's better to allow to use your monad inside more complex ones by making it polymorphic (e.g. forall eff. Eff (console :: CONSOLE | eff), that means it can do CONSOLE stuff but not limited to be used in context of others);
   • | could be read as as (this aliases whole block inside parentheses).

   For async stuff (kinda threading, but remember you're in js world, it's not really threads) you have similar Aff monad. You also should read docs about this too.

   Few tips about Aff:

   • Doing Aff is kinda doing forkIO in Haskell I believe;
   • Use launchAff or launchAff_ to run Aff from Eff monad asynchronously;
   • Use forkAff to run another Aff from Aff monad asynchronously;
   • Use liftEff (Control.Monad.Eff.Class from purescript-eff) to execute Eff from Aff monad;
   • Use liftEff' (Control.Monad.Aff from purescript-aff) to execute Eff from Aff monad if Eff monad has EXCEPTION effect.

   Keep in mind that PureScript is strict by default, so using:

   if condition
      then someMonad foo bar
      else pure unit

   could be better than:

   when condition $ someMonad foo bar

   in sense of efficiency, because if condition compiles to native js if condition while when constructs function reference with possible partial application.

   See also:

   • https://pursuit.purescript.org/packages/purescript-eff
   • https://pursuit.purescript.org/packages/purescript-aff

9. About booleans

   PureScript	Haskell
   Type Boolean	Type Bool
   Value true	Value True
   Value false	Value False

10. About tuples

    In PureScript there's no special syntax for tuples.

    You also need to install purescript-tuples.

    PureScript	Haskell
    Type Tuple Bool Int	Type (Bool, Int)
    Value Tuple true 42	Value (True, 42)
    Pattern (Tuple x y)	Pattern (x, y)

11. About lists

    PureScript has builtin Arrays. Functional Lists are provided by purescript-lists package.

    [1,2,3] will produce an Array Int (not [Int] because in PureScript there's no sugar for typing Arrays/Lists).

    PureScript doesn't have special syntax for Array comprehensions.
    Here is an example of doing comprehension using monads:

    factors :: Int -> Array (Tuple Int Int)
    factors n = do
      a <- 1 .. n
      b <- 1 .. a
      guard $ a * b == n
      pure $ Tuple a b

    An example of Array patterns:

    f []     = -1
    f [x]    = x
    f [x, y] = x * y
    f _      = 0

    There's no builtin cons for Arrays for pattern-matching (some performance issues) but some helpers are provided by purescript-arrays package.

    See also about this:
    https://stackoverflow.com/questions/42450347/purescript-pattern-match-arrays-of-unknown-length#42450443

    Pattern-matching on Lists:

    PureScript	Haskell
    (Cons x xs)	(x : xs)

12. About records:

    Records in PureScript isn't limited to be used in context of data, they're independent, you don't need (but may) have a wrapper for a record.

    Here is an example of a function that works with records:

    foo :: { foo :: String, bar :: Int } -> Int
    foo x = x.bar

    Type of foo is equivalent to:

    foo :: Record (foo :: String, bar :: Int) -> Int

    foo also can deal with any record that have bar :: Int if it's typed like this:

    foo :: forall r. { bar :: Int | r } -> Int

    You can read about | above, it acts here the same way.

    Constructing new records is simple:

    bar = { foo: "Foo", bar: 42, baz: true }

    But keep in mind that when you construct new record you use : but when you update a record you use =:

    bar { bar = 34 }

    An example how to update a nested record:

    foo { bar { baz { bzz = 42 } } }

    Destructuring also works as in js:

    1. foo x = log x.bar

       foo { bar } = <- log bar

       foo { bar: baz } = <- log baz

    2. foo = do
         x <- bar
         log x.baz

       foo = do
         { baz } <- bar
         log baz

       foo = do
         { baz: bzz } <- bar
         log bzz

13. About deriving type class instance:

    Deriving instances separated from data, here's an example:

    derive instance eqLocation :: Eq Location
    derive instance genericLocation :: Generic Location
    instance showLocation :: Show Location where show = gShow

    Names eqLocation, genericLocation and showLocation is just for produced js code, they're named like this just by convention but they can be named differently.

14. About imports:

    In PureScript you don't have qualified keyword for imports, if an import have as alias it is qualified.

    In PureScript as keyword must be places after everything (even after explicit imports).

    PureScript	Haskell
    import Data.Foo as Foo	import qualified Data.Foo as Foo
    import Data.Foo as Foo (foo)	import qualified Data.Foo (foo) as Foo

15. About some packages:

    • Maybe isn't included, install purescript-maybe
    • purescript-console for writing to the console
    • purescript-nullable to deal with js nulls (when you really need it)
    • purescript-generics to deal with Generic stuff
    • purescript-lens if you're looking for Kmett's lenses

This is pretty short list that supposed to get basic stuff as fast as possible, read articles by this links to go deeper:

• https://github.com/purescript/documentation/blob/master/language/Differences-from-Haskell.md
• https://github.com/purescript/documentation/blob/master/guides/Getting-Started.md
• https://github.com/purescript/documentation/tree/master/language