Learn Purescript in Y minutes

purescript.html.markdown

@@ -0,0 +1,195 @@
+---
+language: purescript
+contributors:
+    - ["Fredrik Dyrkell", "http://www.lexicallyscoped.com"]
+---
+
+PureScript is a small strongly, statically typed language compiling to Javascript.
+
+* Learn more at [http://www.purescript.org/](http://www.purescript.org/)
+* Documentation: [http://docs.purescript.org/en/latest/](http://docs.purescript.org/en/latest/)
+* Book: Purescript by Example, [https://leanpub.com/purescript/](https://leanpub.com/purescript/)
+
+```haskell
+
+--
+-- 1. Primitive datatypes that corresponds to their Javascript
+-- equivalents at runtime.
+
+-- Numbers
+1 + 7*5 :: Number -- 36
+-- Types are inferred, so the following works fine
+9 / 2.5 + 4.4 -- 8
+-- Hexadecimal literals
+0xff + 1 -- 256
+-- Unary negation
+6 * -3 -- -18
+6 * negate 3 -- -18
+-- Modulus
+3 % 2 -- 1
+4 % 2 -- 0
+-- Inspect the type of an expression in psci
+:t 9 / 2.5 + 4.4 -- Prim.Number
+
+-- Booleans
+true :: Boolean -- true
+false :: Boolean -- false
+-- Negation
+not true --false
+23 == 23 -- true
+1 /= 4 -- true
+1 >= 4 -- false
+-- Comparisions < <= > >=
+-- are defined in terms of compare
+compare 1 2 -- LT
+compare 2 2 -- EQ
+compare 3 2 -- GT
+-- Conjunction and Disjunction
+true && (9 >= 19 || 1 < 2) -- true
+
+-- Strings
+"Hellow" :: String -- "Hellow"
+-- Multiline string
+"Hellow\
+\orld" -- "Helloworld"
+-- Concatenate
+"such " ++ "amaze" -- "such amaze"
+
+--
+-- 2. Arrays are Javascript arrays, but must be homogeneous
+
+[1,1,2,3,5,8] :: [Number] -- [1,1,2,3,5,8]
+[true, true, false] :: [Boolean] -- [true,true,false]
+-- [1,2, true, "false"] won't work
+-- `Cannot unify Prim.Number with Prim.Boolean`
+-- Cons (prepend)
+1 : [2,4,3] -- [1,2,4,3]
+
+-- Requires purescript-arrays (Data.Array)
+-- and purescript-maybe (Data.Maybe)
+
+-- Safe access return Maybe a
+head [1,2,3] -- Just (1)
+tail [3,2,1] -- Just ([2,1]) 
+init [1,2,3] -- Just ([1,2])
+last [3,2,1] -- Just (1)
+-- Random access - indexing
+[3,4,5,6,7] !! 2 -- Just (5)
+-- Range 
+1..5 -- [1,2,3,4,5]
+length [2,2,2] -- 3
+drop 3 [5,4,3,2,1] -- [2,1]
+take 3 [5,4,3,2,1] -- [5,4,3]
+append [1,2,3] [4,5,6] -- [1,2,3,4,5,6]
+
+--
+-- 3. Records are Javascript objects, with zero or more fields, which
+-- can have different types
+let book = {title: "Foucault's pendulum", author: "Umberto Eco"}
+-- Access properties
+book.title -- "Foucault's pendulum"
+
+getTitle b = b.title
+-- Works on all records with a title (but doesn't require any other field)
+getTitle book -- "Foucault's pendulum"
+getTitle {title: "Weekend in Monaco", artist: "The Rippingtons"} -- "Weekend in Monaco"
+-- Update a record
+changeTitle b t = b {title = t}
+changeTitle book "Ill nome della rosa" -- {title: "Ill nome della
+  -- rosa", author: "Umberto Eco"}
+
+--
+-- 4. Functions
+sumOfSquares x y = x*x+y*y
+sumOfSquares 3 4 -- 25
+-- In psci you have to write `let` in front of the function to get a
+-- top level binding
+mod x y = x % y
+mod 3 2 -- 1
+-- Infix application of function
+3 `mod` 2 -- 1
+
+-- function application have higher precedence than all other
+-- operators
+sumOfSquares 3 4 * sumOfSquares 4 5 -- 1025
+
+-- Conditional
+abs' n = if n>=0 then n else -n
+abs' (-3) -- 3
+
+-- Guarded equations
+abs n | n >= 0    = n
+      | otherwise = -n
+
+-- Pattern matching
+
+-- Note the type signature, input is an array of numbers The pattern
+-- matching destructures and binds the array into parts
+first :: [Number] -> Number
+first (x:_) = x
+first [3,4,5] -- 3
+second :: [Number] -> Number
+second (_:y:_) = y 
+second [3,4,5] -- 4
+sumTwo :: [Number] -> [Number]
+sumTwo (x:y:rest) = (x+y) : rest 
+sumTwo [2,3,4,5,6] -- [5,4,5,6]
+
+-- sumTwo doesn't handle when the array is empty or just have one
+-- element in which case you get an error
+sumTwo [1] -- Failed pattern match
+
+-- Complementing patterns to match
+-- Good ol' Fibonacci
+fib 1 = 1
+fib 2 = 2
+fib x = fib (x-1) + fib (x-2)
+fib 10 -- 89
+
+-- Use underscore to match any, where you don't care about the binding name
+isZero 0 = true
+isZero _ = false
+
+-- Pattern matching on records
+ecoTitle {author = "Umberto Eco", title = t} = Just t
+ecoTitle _ = Nothing
+
+ecoTitle book -- Just ("Foucault's pendulum")
+ecoTitle {title: "The Quantum Thief", author: "Hannu Rajaniemi"} -- Nothing
+-- ecoTitle requires both field to type check:
+ecoTitle {title: "The Quantum Thief"} -- Object does not have property author
+
+-- Lambda expressions
+(\x -> x*x) 3 -- 9
+(\x y -> x*x + y*y) 4 5 -- 41 
+sqr = \x -> x*x
+
+-- Currying
+add x y = x + y -- is equivalent with
+add = \x -> (\y -> x+y)
+add3 = add 3
+:t add3 -- Prim.Number -> Prim.Number
+
+-- Forward and backward function composition
+-- drop 3 followed by taking 5
+(drop 3 >>> take 5) (1..20) -- [4,5,6,7,8]
+-- take 5 followed by dropping 3
+(drop 3 <<< take 5) (1..20) -- [4,5]
+
+-- Operations using higher order functions
+even x = x % 2 == 0
+filter even (1..10) -- [2,4,6,8,10]
+map (\x -> x+11) (1..5) -- [12,13,14,15,16]
+
+-- Requires purescript-foldable-traversabe (Data.Foldable)
+
+foldr (+) 0 (1..10) -- 55
+sum (1..10) -- 55
+product (1..10) -- 3628800
+
+-- Testing with predicate 
+any even [1,2,3] -- true
+all even [1,2,3] -- false
+
+```
+