[Haskell-cafe] Confusing type specialisation in ghci

[David Carter]

Isaac Dupree wrote:

> David Carter wrote:
>
>> This is all as I would expect so far, but:
>>
>> Prelude> let sqlist = map sq
>> Prelude> :t sqlist
>> sqlist :: [Integer] -> [Integer]
>>
>> And indeed, I get
>>
>> Prelude> sqlist [2.5]
>>
>> <interactive>:1:8:
>>    No instance for (Fractional Integer)
>>    ... etc
>
>
> The dreaded Monomorphism Restriction, which GHCi chooses to apply 
> after every line you input.  You could say "let sqlist a = map sq a" 
> (syntactically having arguments makes the defaulting of (Num a => [a] 
> -> [a]) not apply), or just use `ghci -fno-monomorphism-restriction`.
>
> Isaac
>
Thanks -- I had a feeling I'd seen something about it before, but 
couldn't remember where.

Anyway it's good to know the syndrome has a name and a prevention 
strategy, even if not a cure :-).

David


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