WARNING pragmas in `Prelude.undefined` and `Prelude.error`

[Edward Kmett]

The idiom of using undefined to replace BangPatterns has a few benefits,
which is why it gets used in libraries like bytestring. It doesn't require
a language extension and applies a bit nicer in macro expansion because it
infects one line of your patterns rather than either mangling all pattern
matches or relying on subtleties about whether or not the pattern match
earlier was enough to 'forget' the strictness of a later pattern when it
isn't strict in a subsequent pattern.

foo !(Bar x) !(Bar y) = ...
foo y w = ... am I strict in w?

foo x y | seq x (seq y False) = undefined
foo (Bar x) (Bar y) = ...
foo y w = ... is definitely strict in w

Other usecases are for working with combinators like sizeOf, alignment,
bitSize, bitSizeMaybe, finiteBitSize, or doing things like initializing
self-referential IORef structures whenever you do have to tie the knot
strictly, etc. In the former cases you can argue that the API should change
to take a Proxy, then spend a couple of years driving that change process
through, as most of them affect the current language report, but in the
latter case you're basically stuck with either embracing momentary
partiality and knowledge of lack of escape or working with inaccurately
weak types after the structure is initialized, forever.

-Edward

On Mon, Dec 23, 2019 at 11:30 AM Vilem Liepelt <vliepelt at futurefinance.com>
wrote:

>
> > On 22 Dec 2019, at 14:15, Zemyla <zemyla at gmail.com> wrote:
> >
> > The most common legitimate usage for undefined I have is strictness:
> >
> > func x y | x `seq` False = undefined
> > func x y = ...
> >
> > There's nothing else that would go there, and when it's compiled, the
> reference to undefined disappears.
>
> Is this (better than | different from) using BangPatterns?
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