Command: `stack ghc grad.hs -- -Wtype-defaults`

grad.hs

{-# LANGUAGE AllowAmbiguousTypes, RankNTypes, NoMonomorphismRestriction, ConstraintKinds #-}

-- Minimal AD example with polymorphism.

import Numeric.AD

type Autofloat a = (RealFloat a, Floating a, Real a, Show a, Ord a)

f :: (Autofloat a) => [a] -> a
f [x, y] = x^2 + y^2 + 0.1

appGrad :: (Autofloat a) => (forall b . (Autofloat b) => [b] -> b) -> [a] -> [a]
appGrad f l = grad f l

state :: (Autofloat a) => [a]
state = [1.10, -1.2]

res = appGrad f state

main :: IO ()
main = putStrLn $ show $ res