Just to clarify, I know it was my mistake, and so I'm not blaming Haskell or Ghc. The first few times you realise the compiler isn't a magic wand that stops you being silly are the hardest.

Jamie Love wrote:

Oh, I see

I wasn't thinking through the code (and I'm still in the honeymoon phase with Haskell, thinking it can do no wrong).

Don Stewart wrote:

jamie.love:

   Ah, of course.

   Thanks. I removed the hPut and it runs smoothly.  I had forgotten that
   haskell chooses the types dynamically.

   Shouldn't haskell pick up that there is no 'mod' for Word8?  I mean,
   shouldn't I get a nicer error message?


Well, it inferred Word8 for your generated values, so 256 overflowed to 0.
Stating the expected type here would prevent that. (And is why mandatory 
top level declarations are good -- they can prevent bugs caused by 
an unexpected type being inferred).

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