I have the following custom prelude:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE RankNTypes #-}
module Prelude (
module ClassyPrelude
, module Numeric.Units.Dimensional.Prelude
, headMay
, sec
) where
import ClassyPrelude hiding
( (+), (-), (*), (/), (^), (**)
, abs, signum, negate, recip, pi, exp, log, logBase, sqrt
, sin, cos, tan, asin, acos, atan, atan2
, sinh, cosh, tanh, asinh, acosh, atanh
, sum, product, minimum, maximum
) -- Hide definitions overridden by 'Numeric.Dimensional'.
import Numeric.Units.Dimensional.Prelude hiding (
(++)
, all,and, any, break, concat, concatMap, drop, dropWhile, elem, filter, foldMap, foldr
, getChar, getContents, getLine, head, init, interact, lookup, map, mapM_, notElem, null, or, print, putChar
, putStr, putStrLn, readFile, replicate, sequence_, splitAt, undefined, writeFile
, last, length, lines, reverse, second, span, tail, take, takeWhile, unlines, unwords
, unzip, unzip3, words, zip, zip3, zipWith, zipWith3
) -- hide other conflicts specific to classy prelude
import qualified Numeric.Units.Dimensional.Prelude as NUDP
headMay :: Foldable f => f a -> Maybe a
headMay = foldr (const . Just) Nothing
sec :: Num a => Unit Metric DTime a
sec = NUDP.second
The relevant bit here is just (I think) Classy Prelude and the definition of headMay:
headMay :: Foldable f => f a -> Maybe a
headMay = foldr (const . Just) Nothing
The error I get when trying to compile this is:
/prelude/Prelude.hs:32:18: error:
• Couldn't match type ‘a’ with ‘Element (f a)’
‘a’ is a rigid type variable bound by
the type signature for:
Prelude.headMay :: forall (f :: * -> *) a.
Foldable f =>
f a -> Maybe a
at prelude/Prelude.hs:31:1-39
Expected type: Element (f a) -> Maybe a -> Maybe a
Actual type: a -> Maybe a -> Maybe a
• In the first argument of ‘foldr’, namely ‘(const . Just)’
In the expression: foldr (const . Just) Nothing
In an equation for ‘Prelude.headMay’:
Prelude.headMay = foldr (const . Just) Nothing
• Relevant bindings include
headMay :: f a -> Maybe a (bound at prelude/Prelude.hs:32:1)
|
32 | headMay = foldr (const . Just) Nothing
| ^^^^^^^^^^^^
classy-prelude exports a MonoFoldable version of foldr, through the reexport of Data.MonoTraversable.Unprefixed: