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

{- Result:
grad.hs:10:12: warning: [-Wtype-defaults]
    • Defaulting the following constraints to type ‘Integer’
        (Integral b0) arising from a use of ‘^’ at grad.hs:10:12-14
        (Num b0) arising from the literal ‘2’ at grad.hs:10:14
    • In the first argument of ‘(+)’, namely ‘x ^ 2’
      In the first argument of ‘(+)’, namely ‘x ^ 2 + y ^ 2’
      In the expression: x ^ 2 + y ^ 2 + 0.1

grad.hs:10:18: warning: [-Wtype-defaults]
    • Defaulting the following constraints to type ‘Integer’
        (Integral b0) arising from a use of ‘^’ at grad.hs:10:18-20
        (Num b0) arising from the literal ‘2’ at grad.hs:10:20
    • In the second argument of ‘(+)’, namely ‘y ^ 2’
      In the first argument of ‘(+)’, namely ‘x ^ 2 + y ^ 2’
      In the expression: x ^ 2 + y ^ 2 + 0.1

grad.hs:21:19: warning: [-Wtype-defaults]
    • Defaulting the following constraints to type ‘Double’
        (Show a0) arising from a use of ‘show’ at grad.hs:21:19-28
        (RealFloat a0) arising from a use of ‘res’ at grad.hs:21:26-28
    • In the second argument of ‘($)’, namely ‘show $ res’
      In the expression: putStrLn $ show $ res
      In an equation for ‘main’: main = putStrLn $ show $ res
-}