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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

1. 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.

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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Operating system	Unknown
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