In GHC 6.0.1, using GHCi:
Prelude let x # f = f x
Prelude :t (#)
interactive:1: parse error on input `)'
Prelude :t ( #)
interactive:1: parse error on input `#)'
Prelude :t (# )
interactive:1: parse error on input `)'
Prelude :t ( # )
( # ) :: forall t t1. t1 - (t1 - t) - t
Prelude zipWith (#)
In GHC 6.0.1, using GHCi:
Prelude let x # f = f x
Prelude :t (#)
interactive:1: parse error on input `)'
Prelude :t ( #)
interactive:1: parse error on input `#)'
Prelude :t (# )
interactive:1: parse error on input `)'
Prelude :t ( # )
( # ) :: forall t t1. t1 - (t1 - t) - t
Prelude