[Haskell-cafe] Confusing type specialisation in ghci

David Carter dmc at sanger.ac.uk
Mon Oct 8 04:26:12 EDT 2007

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 :-).


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