I'm observing an interesting behavior when using pattern matching with pattern guards and all warnings turned on
{-# OPTIONS_GHC -Wall #-}
module Mood where
data Mood = Happy
| Indifferent
| Sad
deriving Show
flipMood :: Mood -> Mood
flipMood Happy = Sad
flipMood Indifferent = Indifferent
flipMood Sad = Happy
flipMood' :: Mood -> Mood
flipMood' mood
| Happy <- mood = Sad
| Indifferent <- mood = Indifferent
| Sad <- mood = Happy
Even though flipMood and flipMood' are pretty much doing the same thing I get the following error message:
Mood.hs:15:1: Warning:
Pattern match(es) are non-exhaustive
In an equation for ‘flipMood'’: Patterns not matched: _
Ok, modules loaded: Mood.
and therefore need to add a catch all case like
| otherwise = mood
to satisfy the exhaustiveness checker.
Core seems to be just fine with these two functions behaving the same:
flipMood =
\ ds_dTh ->
case ds_dTh of _ {
Happy -> Sad;
Indifferent -> Indifferent;
Sad -> Happy
}
flipMood' = flipMood
With optimizations turned off I get the following Core output which seems to explain this behavior:
flipMood' =
\ mood_axV ->
case mood_axV of wild_X9 {
__DEFAULT ->
case wild_X9 of _ {
Indifferent -> Indifferent;
Sad -> Happy
};
Happy -> Sad
}
Why is it behaving this way? Am I missing something?
Kind regards, raichoo