-- the identity function
Id x = x

-- eager version of `if cond then a else b`
Choice True a _ = a
Choice False _ b = b

-- function composition
(f . g) x = f (g x)

-- reverse the order of the arguments of f
Flip f x y = f y x

-- Fixed-point combinator
-- https://www.wikiwand.com/en/Fixed-point_combinator
Fix f x = f (Fix f) x

-- if cond then a else b
IfThenElse True a? _? = a
IfThenElse False _? b? = b

-- instantiates a thunk
Unthunk t = if IsThunk t then t () else t