Bang patterns bind... unexpectedly. Deserves loud warning

Consider the following,

{-# LANGUAGE BangPatterns #-}
hello :: [Int] -> ()
hello (!x:xs) = ...

To me it seems like the bang pattern should be applied to x; but it's not! In fact, this program is equivalent to,

hello (!(x:xs)) = ...

This strikes me as quite surprising, yet there is no mention of how ! is supposed to bind in the users guide nor the Haskell Prime proposal.

This behavior is slightly less surprising if you consider that the bang pattern !Just x is equivalent to !(Just x) (although this also strikes me as a bit unfortunate, although it is likely too late to do much about this).

In sum: We need to document the current syntactic binding properties of ! in the users guide.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Documentation
Test case
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changed milestone to %8.2.1

changed weight to 5

added Tbug Trac import documentation labels

I think it would be better to make bang bind more tightly so that it behaves the way you expect. We go to some trouble to make

f !x y = e

mean

f (!x) y = e

so bang binds pretty tightly there.

This is a purely syntactic matter. Would anyone like to tackle it?

Simon

changed milestone to %8.4.1

Given that 8.2.1-rc1 is imminent, I'm bumping these off to the 8.4

changed milestone to %8.6.1

This ticket won't be resolved in 8.4; remilestoning for 8.6. Do holler if you are affected by this or would otherwise like to work on it.

changed milestone to %8.8.1

These won't be fixed for 8.6, bumping to 8.8.

changed milestone to %8.10.1

Bumping milestones of low-priority tickets.

added Pnormal label

removed milestone

Apparently this was fixed in GHC 9.0.1, for example let (!x : xs) = [undefined] in xs now throws an exception.

Related: (x : !xs) didn't compile before, but works now, with the expected behaviour.

OK great! Would someone feel able to add regression tests? Thanks.