[Haskell-cafe] Confusing type specialisation in ghci
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]
>> 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`.
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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