Where can BangPatterns appear

Question

From GHC user guide it seems like most Pat can be PBangPat, but there are a few exceptions. e.g. top-level bangs in a module (like !main) aren't allowed and x : !xs fails to parse x : (!xs) parses thanks @chi. What is the formal specification about where the bangs can be added? I've looked into some chapters of the user guide and the Report but found nothing.

Answer 1

There is no accepted formal specification for BangPatterns since they are not a part of any Haskell Report. The closest thing we have to a specification is the User's Guide along with the haskell-prime proposal it links to.

Both of those sources explicitly mention that a bang pattern is not allowed at the top level of a module.

As for x : !xs, the User's Guide has this to say about the syntax of bang patterns:

We add a single new production to the syntax of patterns:

pat  ::= !pat

It should be read in conjunction with the Haskell 2010 Report:

pat  ::= lpat qconop pat
       | lpat

lpat ::= apat
       | - (integer | float)
       | gcon apat_1 ... apat_k

apat ::= var [ @ apat]
       | ...
       | ( pat )
       | ...

According to these rules x : !xs actually should parse (since !xs is a pat, the whole thing is lpat qconop pat). So either the User's Guide (and the haskell-prime proposal) is wrong or GHC is wrong on this point.

I believe that in practice the syntax accepted by GHC is "anything that looks like a valid expression" including interpreting (!x) as a section of the operator !. For example (! Just x) is accepted as a pattern but (! ! x) is not.