Template Haskell's TupleT considers unary tuples as a valid types.

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:

tupE [exp] behaves as expected:

Prelude Language.Haskell.TH> $(sigE (tupE [[|1|]]) (conT ''Int))
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
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown

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