Re: [Haskell-beginners] let in GHCI

Thanks a lot David - that looks like it. Searched 'monomorphism' and found some material in the Haskell wiki to read. Wasn't galloping senility at least, in this case.. Cheers/ Henry On 11 Dec 2011, at 18:36, David McBride wrote:

I don't understand it perse, but I've run into it often enough to say that the problem is the monomorphism restriction. When you say let mySort = sort, instead of having the type

mySort :: Ord a => [a] -> [a]

which is what you'd expect, instead there is some bizarre rule in the haskell spec that says it has to choose () for a in this case so if you look at the type after you define it in ghci, you'll see the type is actually

mySort :: [()] -> [()]

which is completely useless for anything. If you go :set -XNoMonomorphismRestriction in ghci, you'll get the correct type and it will work the way you intend. There is apparently some obscure case where this is desirable and that is the only reason it has not been removed by default.

On Sun, Dec 11, 2011 at 1:14 PM, Henry Lockyer wrote:

...

Hello Haskellers

Why is it that in GHCI I can do, for example,

...

replicate 6 5 [5,5,5,5,5,5] let my6 = replicate 6 my6 5 [5,5,5,5,5,5]

but if I do

...

sort "bav" "abv"

this is ok, but

...

let mySort = sort mySort "bav"

<interactive>:1:8: Couldn't match expected type `()' with actual type `Char' Expected type: [()] Actual type: [Char] In the first argument of `mySort', namely `"bav"' In the expression: mySort "bav"

and/or

...

mySort [6,5,9]

<interactive>:1:13: No instance for (Num ()) arising from the literal `9' Possible fix: add an instance declaration for (Num ()) In the expression: 9 In the first argument of `mySort', namely `[6, 5, 9]' In the expression: mySort [6, 5, 9]

This is eluding me at the moment..! ;-)

/ Henry

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