[GHC] #2358: Template Haskell's TupleT considers unary tuples as a valid types.

[GHC]

#2358: Template Haskell's TupleT considers unary tuples as a valid types.
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    Reporter:  fons     |       Owner:                  
        Type:  bug      |      Status:  new             
    Priority:  normal   |   Component:  Template Haskell
     Version:  6.8.2    |    Severity:  normal          
    Keywords:           |    Testcase:                  
Architecture:  Unknown  |          Os:  Unknown         
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 I don't know how tuples are internally implemented in GHC, but the Haskell
 report (3.8) specifically establishes two as their lower size bound.

 When provided a zero (or null list) arguments, all TupleT, TupP and TupE
 behave like the unit type (which is certainly reasonable). However, they
 handle argument 1 (or the singleton list) in different ways:

  * {{{TupleP [pat]}}} is equivalent to {{{pat}}} (which again, is
 perfectly reasonable)
  * {{{TupE [exp]}}} behaves as exp (again, it seems right)

 However:

  * {{{TupleT 1 `AppT` VarT ''Int}}} is a different type than {{{VarT
 ''Int}}}

 See the following two examples:

  1. {{{tupE [exp]}}} behaves as expected:

 {{{
 Prelude Language.Haskell.TH> $(sigE (tupE [[|1|]]) (conT ''Int))
 1
 }}}

  2. {{{tupleT 1}}} doesn't:

 {{{
 Prelude Language.Haskell.TH> $(sigE [|1|] (tupleT 1 `appT` conT ''Int))

 <interactive>:1:2:
     No instance for (Num (Int))
       arising from the literal `1' at <interactive>:1:2-40
     Possible fix: add an instance declaration for (Num (Int))
     In the expression: $(sigE [| 1 |] (tupleT 1 `appT` conT 'Int))
     In the definition of `it':
         it = $(sigE [| 1 |] (tupleT 1 `appT` conT 'Int))
 }}}

 Probably, since unary tuples are not legal, the typechecker should simply
 raise an error when {{{TupleT}}} is found to have received {{{1}}} as
 argument.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2358>
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