[GHC] #2806: Require bang-patterns for unlifted bindings

[GHC]

#2806: Require bang-patterns for unlifted bindings
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    Reporter:  simonpj           |        Owner:                  
        Type:  feature request   |       Status:  new             
    Priority:  normal            |    Milestone:  6.12.1          
   Component:  Compiler          |      Version:  6.8.3           
    Severity:  normal            |   Resolution:                  
    Keywords:                    |   Difficulty:  Unknown         
    Testcase:                    |           Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |  
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Comment (by guest):

 darn, I (Isaac) realized that what I said was really ambiguous.  I'll try
 with simpler examples:

 Perhaps

 {{{
 f (I# z) = ...
 }}}

 ought not to require a bang on the "z", because the data-type is
 inherently strict.

 And to make it clear, another example of this logic is that this would be
 okay (but needs that bang) :
 {{{
 g (x, !(I# z)) = ...
 }}}

 But, I forget the semantics of bang-patterns... would this also be
 equivalent and permissible?:
 {{{
 g (x, (I# !z))
 }}}

 Anyway, if we allow "f" above (or even if not?), I would be more
 comfortable with -- by analogy -- requiring a bang in any data-type
 definition where an unlifted type is contained, rather than leaving that
 form of strictness implied.  (In the presence of type-synonyms, it may not
 always be obviously visible to the reader whether a member type is
 unlifted.)  This makes a difference in currently-allowed expressions like
 `I# ( 1# +# 1# )` or `I# ( 1# `quot#` 0# )` which would "normally" be lazy
 as long as there isn't a bang in the data definition of Int (but is strict
 because of unlifted semantic magic).

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2806#comment:7>
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