[Haskell-cafe] Beginner question

Supposing I have the following code:

module Main(main) where main = putStr (show [])

I will get these errors from GHC and Hugs respectively:

Main.hs:2:16: Ambiguous type variable `a' in the constraint: `Show a' arising from a use of `show' at Main.hs:2:16-22 Probable fix: add a type signature that fixes these type variable(s)

ERROR "src/Main.hs":2 - Unresolved top-level overloading *** Binding : main *** Outstanding context : Show b

But if I change my code to the following, it will compile.

module Main(main) where main = putStr (show [1])

I have no problems typing in "putStr (show [])" in Hugs... runs fine. So what's wrong? I've really tried hard to think of an explanation, but with no luck.. Thanks, Benjamin