[Haskell-cafe] ghc should be able to deduce correct use of partial functions and not give a warning in such cases

[Olaf Klinke]

> ghc should be able to deduce correct use of partial functions and not
> give
> a warning in such cases, e.g. in 9.8.2
> 
>      if null ys then [] else [(xs,tail ys)])
> 
> gets a warning
> 
>       warning: [GHC-63394] [-Wx-partial]
> 
> but it is clear that this use of tail will never be a problem so IMHO
> that
> line of code should not get a warning.
> 
> Does anybody know if there is a plan or enhancement request to
> eliminate
> such warnings?
> 
> Cheers
> George

The problem is that both partiality checks and checks for incomplete
pattern matches are undecidable in general, that is, without assistance
the compiler can not infer partiality without risking infinite loops. 
Checks for incomplete pattern matches could be decidable, if it was not
for view patterns, where you can hide arbitrary function applications
(think unsafePerformIO) in. Currently GHC plays the safe card, see this
thread:
https://mail.haskell.org/pipermail/haskell-cafe/2024-January/136545.html

With the help of special annotation such as Liquid Haskell you might be
able to teach the compiler that your use of partial functions is fine. 
https://ucsd-progsys.github.io/liquidhaskell/

However, I second Henning in that a -Wx-partial warning hints at the
necessity of a code re-factoring. 

Olaf