Re: [Haskell-cafe] Haskell Debugging

On 14/11/06, Cale Gibbard wrote:

On 14/11/06, Valentin Gjorgjioski wrote:

...

Just one more thing

If I write

ex9 :: [Float] ex9 = (observe "after reverse" ) reverse [10.0,7.0,3.0,0.0,4.0]

it doesn't work. If I delete ex9 :: [Float] then it works fine. any suggestions?

This doesn't happen for me. The only thing I can think of trying is to check the type of ex9 by deleting the type signature, loading the file, and typing: :t ex9 on the hugs prompt. If it prints anything other than ex9 :: [Float] then you'll have your answer.

- Cale

Sorry, I just realised what you meant, and I do get the behaviour you initially described without the typesignature there. Actually, without the typesignature, it's inferring the type [Double], due to defaulting. However, adding the typesignature ex9 :: [Double] will again cause it to print the values in the list rather than raw thunks. So perhaps it's because you haven't given a typesignature explicitly, so the list is initially polymorphic, which means that the values in it are actually function calls initially (that is, things like (fromRational 10.0)), and they only end up getting a specialised type later, when hugs finishes compiling the module and applies the monomorphism restriction and defaulting, but hugs isn't going back and optimising the function with the additional knowledge? I don't really know, that's my best guess at it. Maybe someone who knows hugs better would know with more certainty? - Cale