Simple EDSL in PureScript

Main.purs

module Main where

import Data.Traversable (traverse_)
import Effect (Effect)
import Effect.Console (logShow)
import Prelude

data Expression
  = Num Int
  | Add Expression Expression
  | Subtract Expression Expression

eval :: Expression -> Int
eval (Num a) = a
eval (Add a b) = (eval a) + (eval b)
eval (Subtract a b) = (eval a) - (eval b)

expressions :: Array Expression
expressions =
  [ Num 1                                  -- 1
  , Add (Num 1) (Num 2)                    -- 1 + 2
  , Subtract (Add (Num 1) (Num 2)) (Num 2) -- (1 + 2) - 2
  ]

main :: Effect Unit
main = traverse_ (logShow <<< eval) expressions