PureScript Cheat Sheet For JavaScript Developers
Literal
Number
JS
let a = 1
let b = 1.2
PureScript
a :: Int
a = 1
b :: Number
b = 1.2
String
JS
let a = "yay"
let multiline = `first line
second line
third line`
let concatString = "a" + "b"
PureScript
a :: String
a = "yay"
multiline = """first line
second line
third line"""
concatString = "a" <> "b"
Boolean
JS
let a = true
let b = false
PureScript
a :: Boolean
a = true
b :: Boolean
b = false
Records
JS
let a = {key1: "value1", key2: true}
let b = Object.assign({}, a, {key1: "new value1"})
function updateKey2To(record) {
return (value) => Object.assign({}, record, {key2: value})
}
PureScript
a :: {key1:: String, key2:: Boolean}
a = {key1: "value1", key2: true}
b = a {key1 = "new value1"}
updateKey2To = _ {key2 = _}
Function
JS
function sum(x, y) { return x + y }
let lambdaSum = (x, y) => (x + y)
function fib(n) {
if(n == 0) return 1
else if(n == 1) return 1
else return fib(n-1) + fib(n-2)
}
PureScript
sum :: Int -> Int -> Int
sum x y = x + y
lambdaSum :: Int -> Int -> Int
lambdaSum = \x y -> x + y
fib :: Int -> Int
fib 0 = 1
fib 1 = 1
fib n = fib (n-1) + fib (n-2)
Control Flow
If-Else
JS
let tof = true ? true : false
PureScript
tof :: Boolean
tof = if true then true else false
Scope
JS
let a = 1
function inScope() {
let a = 2
return a
}
PureScript
a = 1
inScope = a
where a = 2
-- or
inScope = let a = 2
in a
Pattern Matching
JS ES proposal
let res = {status: 404}
let entity = case (res) {
when {status: 200, body: b} ->
b
when {status: s} ->
throw "error" + s
}
PureScript
res :: {status:: Int, body:: String}
res = {status: 404, body: ""}
entity = case res of
{status: 200, body: b} -> Right b
e -> Left $ show e
Do Notaion / FlatMap
JS
JavaScript(ES2019) has flatMap for Array
let a = [1,2,3]
let b = [2,3,4]
let c = a.flatMap(x=> b.flatMap(y => x+y))
PureScript
flatMap in PureScript is the same as Haskell >>=
a = [1,2,3]
b = [2,3,4]
c = a >>= \x -> (b >>= \y -> x + y)
and you can use do notation for nicer syntax
a = [1,2,3]
b = [2,3,4]
c = do
x <- a
y <- b
pure x + y
not only Array, you can flatMap on any kind that has Monad instance i.e. Maybe
Modules
JS
JavaScript modules are based on file directory
// import module from somefile.js
import {method} from './some-file'
// export module
const a = 1
export a
PureScript
While PureScript is namespace based
-- file ramdom-file-name.purs
module A where
a = 1
so just module name matter, the file name doesnot matter
module B where
import A
-- not import './ramdom-file-name'