Syntatic Cheat Sheet

Translating from TypeScript to Haskell
Basics
Comments
TypeScript
Haskell
TypeScript
// This is a TypeScript comment
​
/*
Need more space?
No problem!
This is a multi-line TypeScript comment :)
*/
​
/**
 * In TS, you write documentation like this for TypeDoc
 */
 const id = a => a
Haskell
-- This is a Haskell comment
​
{-
Need more space?
No problem!
This is a multi-line Haskell comment :)
-}
​
--| Add a pipe for a short function description
--  You can keep talking about is afterwards
--  These will turn into formatted documentation with Haddock
id a = a
​
{-| You can use a multi-line comment for Haddock, too
    The same rules apply.
-}
swap (a, b) -> (b, a)
Assignment
Implicit Type
TypeScript
Haskell
TypeScript
const hi = "Hello World"
Haskell
hi = "Hello World"
Explicit Type
TypeScript
Haskell
Haskell (Local Clarification Variant)
TypeScript
const hi: string = 'Hello World'
Haskell
hi :: Text
hi = "Hello World"
Haskell (Local Clarification Variant)
hi = ("Hello World" :: Text)
Functions
Anonymous
TypeScript
Haskell
Haskell (Shorthand)
TypeScript
a => a * 2
Haskell
\a -> a * 2
-- NOTE: `\` is an easier to type lambda `λ`
Haskell (Shorthand)
(*2)
Named
TypeScript
Haskell
Haskell (Shorthand)
TypeScript
const double = (a : number) => a * 2
Haskell
double :: Int -> Int
double a = a * 2
Haskell (Shorthand)
double :: Int -> Int
double = (*2)
Infix
TypeScript
Haskell
TypeScript
// Does not exist
Haskell
(*&) :: Int -> String -> String
num *& str = show num ++ str
-- NOTE: `show` turns values into strings
Argument Application
TypeScript
Haskell
TypeScript
const result = doTheThing(1, 2, ["a", "b"], 1.1)
Haskell
result = doTheThing 1 2 ["a", "b"] 1.1
Side-Effectful
TypeScript
Haskell
TypeScript
fireTheMissiles()
Haskell
fireTheMissiles
Inner Scope
TypeScript
Haskell (let...in)
Haskell (where)
TypeScript
const withInner = (toLog: string): void => {
  const innerValue = 'That is the question!'
  console.log(`${toLog}? Or not ${toLog}? ${innerValue}`)
}
Haskell (let...in)
withInner' toLog =
  let
    innerValue = "That is the question!"
    msg = toLog <> "? Or not " <> toLog <> "? " <> innerValue
  in
    logInfo msg
Haskell (where)
withInner toLog = logInfo msg
  where
    msg = toLog <> "? Or not " <> toLog <> "? " <> innerValue
    innerValue = "That is the question!"
Nested Application
TypeScript
Haskell (parens)
Haskell (applied with $)
Haskell (Piped with &)
TypeScript
const result = triple(prod(1, double(1), 3))
​
/* i.e.
  const two = double(1)
  const six = prod(1, two, 3)
  const result = triple(six)
*/
Haskell (parens)
result = triple (prod 1 (double 1) 3)
Haskell (applied with $)
result = triple $ prod 1 (double 1) 3
Haskell (Piped with &)
result = 3
       & prod 1 (double 1)
       & triple
Composition
TypeScript
Haskell
TypeScript
const timesSix = (a: number): number => triple(double(a))
Haskell
timesSix :: Int -> Int
timeSix = triple . double
Partial Application
TypeScript
Haskell
Haskell (Point-Free Style)
TypeScript
const math = (n: number, m: number): number =
  triple(prod(10, double(1), n), m)
Haskell
math :: Int -> Int -> Int
math n m = triple (prod 10 (double 1) n) m
Haskell (Point-Free Style)
math :: Int -> Int -> Int
math = triple . prod 10 (double 1)
Modules
TypeScript
Haskell
TypeScript
import * from "./GetEverything"
import * as HL from "./HelperLib";
import { foo, bar } from "./SomeLib"
​
const id = a => a
const swap = ([a, b]) => [b, a]
const hiddenValue = HL.downcase("not exported")
​
export {
  id,
  swap
}
​
export { toReExport } from "./ExternalLib"
Haskell
module MyModule
  ( id
  , swap
  , toReExport
  ) where
​
import           GetEverything
import           SomeLib     (foo, bar)
import           ExternalLib (toReExport)
import qualified HelperLib   as HL
​
id a = a
swap (a, b) = (b, a)
hiddenValue = HL.downcase "not exported"
Types & Data
Type Alias
TypeScript
Haskell
TypeScript
type Name = string
type Balance = number
Haskell
type Name    = Text
type Balance = Int
Sum / Enum / Coproduct
TypeScript
Haskell
TypeScript
type TrafficLight = 'RED' | 'YELLOW' | 'GREEN'
Haskell
data TrafficLight
  = Red
  | Yellow
  | Green
Product
TypeScript
Haskell
TypeScript
type GearRatio = [number, number] // [big gear, little gear]
type Bike = [string, GearRatio] // [bikeName, gear ratio]
Haskell
data GearRatio = GearRatio Int  Int
data Bike      = Bike      Text GearRatio
Interfaces & Classes → Records
TypeScript (Interface)
TypeScript (Class)
Haskell
TypeScript (Interface)
interface GearRatio {
  big: number
  little: number
}
​
interface Bike {
  name: string
  gear: GearRatio
}
TypeScript (Class)
class Geared {
  big: number
  little: number
​
  constructor(gbr: number, lgr: number) {
    big = bgr
    little = lgr
  }
}
​
class Bike extends Geared {
  name: string
​
  constructor(bgr: number, lgr: number, bikeName: string) {
    name = bikeName
    super(bgr, lgr)
  }
}
Haskell
data Gear = GearRatio
  { _big    :: Natural
  , _little :: Natural
  }
​
data Bike = Bike
  { _name :: Text
  , _gear :: Gear
  }
Constructors
TypeScript (Direct)
TypeScript (Class)
Haskell
TypeScript (Direct)
const makeGear = (big, little): GearRatio => ({ big, little })
​
const makeBike = (big, little, name): Bike => ({
  name,
  gear = makeGear(big, little)
})
TypeScript (Class)
/*
  Included in the `class` delcaration. For example:
​
  constructor(bgr: number, lgr: number) {
    big = bgr
    little = lgr
  }
*/
Haskell
{-
  Constructors are the capitalized word on
  the right of the `=` in the `data` declaration
  It auto-generates constructor functions
  with the folling type signatures:
​
  GearRatio :: Natural -> Natural   -> Geared
  Bike      :: Text    -> GearRatio -> Bike
-}
Instantiation
TypeScript (Direct)
TypeScript (Class)
Haskell
Haskell (Record Style)
Haskell (Wildcard Style)
TypeScript (Direct)
const myBike = makeBike(2, 5, '10 Speeder')
TypeScript (Class)
const myBike = new Bike(2, 5, '10 Speeder')
Haskell
myBike = Bike "10 Speeder" (GearRatio 2 5)
Haskell (Record Style)
myBike = Bike
  { _name = "10 Speeder"  -- Use named fields
  , _gear = GearRatio 2 5 -- Or just ordered values
  }
Haskell (Wildcard Style)
myBike :: Bike
myBike =
  let
    _big    = 2
    _little = 5
    _name   = "10 Speeder"
    _gear   = GearRatio {..}
  in
    Bike {..}
Getters
TypeScript (Dot Syntax)
TypeScript (Destructuring)
Haskell (Destructuring)
Haskell (Wildcard Style)
Haskell (Lenses)
TypeScript (Dot Syntax)
const big: number = myGears.big
const little = myBike.gear.little
TypeScript (Destructuring)
const { bigGear } = myGears
const getBig = ({ big }) => big
const ratio = ({ big, little }) => big / little
Haskell (Destructuring)
GearRatio big _ = myGears
getBig (GearRatio {_big}) = _big
ratio (GearRatio {_big, _little}) = _big / _little
Haskell (Wildcard Style)
ratio (GearRatio {..}) = _big / _little
Haskell (Lenses)
import Control.Lens.TH
​
$(makeLenses ''Gear)
$(makeLenses ''Bike)
​
myGear ^. big -- NOTE: lenses have no underscores!
​
-- Nested
​
myBike ^. gear . little
Updating with Setters
TypeScript
Haskell (Record Syntax)
Haskell (Lenses)
TypeScript
// Preamble
​
const myGear = new GearRatio(2, 5)
const myBike = new Bike('10 Speeder', myGear)
​
// Mutable Update
​
myGear.littleGear = 6
myBike.name = '12 Speeder'
​
// Immutable Update
​
const upgradedGear = Object.assign({}, myGear)
const upgradedBike = Object.assign({}, myBike, {gear: upgradedGear})
Haskell (Record Syntax)
-- Preamble
​
myGear = GearRatio 2 5
myBike = Bike "10 Speeder" myGear
​
-- Immutable Update
​
upgradedBike = myBike
  { _name = "12 Speeder"
  , _gear = myGear { _littleGear = 6 }
  }
Haskell (Lenses)
-- Preamble
​
import Control.Lens.TH
​
makeLenses ''Gear
makeLenses ''Bike
​
myGear = GearRatio 2 5
myBike = Bike "10 Speeder" myGear
​
-- Immutable Update
​
upgradedBike = myBike & name .~ "12 Speeder"
                      & gear.littleGear .~ 6
Branching
If...Else
TypeScript
TypeScript Ternary
Haskell
TypeScript
if (condition) {
  branchA
} else {
  branchB
}
TypeScript Ternary
condition ? branchA : branchB
Haskell
if condition
   then branchA
   else branchB
If...Else If...Else
TypeScript
Haskell
TypeScript
if (percent >= 90 || student.paidBribe) {
  return GRADES.A
} else if (percent >= 75) {
  return GRADES.B
} else if (percent >= 60) {
  return GRADES.C
} else if (percent >= 50) {
  return GRADES.D
} else {
  return GRADES.F
}
Haskell
if | percent >= 90 || paidBribe student = A
   | percent >= 75 = B
   | percent >= 60 = C
   | percent >= 50 = D
   | otherwise     = F
Switch/Case
TypeScript
Haskell
TypeScript
let nextAction
​
switch (lightState) {
  case LIGHT.RED:
    nextAction = ACTION.STOP
    break
​
  case LIGHT.YELLOW:
    nextAction = ACTION.SLOW
    break
​
  case LIGHT.GREEN:
    nextAction = ACTION.DRIVE
    break
​
  default:
    nextAction = currentAction
}
Haskell
nextAction =
  case lightState of
    Red    -> Stop
    Yellow -> Slow
    Green  -> Drive
    _      -> currentAction
Haskell for TypeScript Developers
Chapter One: Setup
Last updated 6 months ago