haskell-jp / questions #105

ghci> let f = (1 +) :: (Num a) => a -> a
ghci> let g = (2 +) :: (Num a) => a -> a
ghci> :t (f $ g)
(f $ g) :: (Num a, Num (a -> a)) => a -> a

ghci> let a = (1 +) :: Int -> Int
ghci> let b = (2 +) :: Int -> Int
ghci> :t (a $ b)

<interactive>:1:6: error:
    • Couldn't match expected type 'Int' with actual type 'Int -> Int'
    • In the second argument of '($)', namely 'b'
      In the expression: a $ b

Spec.hs 1 1 error could not execute: hspec-discover (lsp)

Selected the snapshot .

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type BenriList :: (k -> Type) -> [k] -> Type
data BenriList f xs where
  BNil :: BenriList f '[]
  BCons :: f x -> BenriList f xs -> BenriList f (x ': xs)
-- f = Identity だと HList と同型