[Haskell-beginners] MultiParamTypeClasses confusion

[Derek McLoughlin]

Thanks for that. I see that I can now use:

ghci> let z  = toVector (y, y) :: (Double, Double)
ghci> z
(10.0,10.0)

On 10 March 2014 13:12, Brandon Allbery <allbery.b at gmail.com> wrote:
> On Mon, Mar 10, 2014 at 8:59 AM, Derek McLoughlin
> <derek.mcloughlin at gmail.com> wrote:
>>
>> ghci> distance (x, x) $ toVector (y, y)
>> 12.727922061357855
>>
>> I don't understand why this doesn't work:
>>
>> ghci> let z = toVector (y, y)
>> interactive>:32:9:
>>     No instance for (Vectorizable (Double, Double) v0)
>>       arising from a use of `toVector'
>>     The type variable `v0' is ambiguous
>>     Possible fix: add a type signature that fixes these type variable(s)
>>     Note: there is a potential instance available:
>>       instance Vectorizable (Double, Double) (Double, Double)
>>         -- Defined at vector.hs:13:10
>>     Possible fix:
>>       add an instance declaration for (Vectorizable (Double, Double) v0)
>>     In the expression: toVector (y, y)
>>     In an equation for `z': z = toVector (y, y)
>>
>> It seems odd that "toVector" works when used as an argument to
>> "distance" but not when used in a let expression.
>>
>> Can anyone explain?
>
>
> When you use them together, Haskell can infer the correct type for `v0` from
> the inferred type of `distance` by applying defaulting: the type of `sqrt`
> introduces a constraint that is subject to defaulting. If you separate them,
> it can no longer determine a concrete type for your `let`, as multiparameter
> type classes are not subject to defaulting (and can't be). Also, Haskell
> cannot conclude from the existence of a single possible instance that it
> should use that instance: the meaning of your expression would then change
> if you imported a module which defined a new instance.
>
> The requirement for a concrete type comes from the monomorphism restriction,
> which is applied to your `let` because `z` has no parameters.
>
> http://www.haskell.org/haskellwiki/Monomorphism_restriction
>
> Many people turn off the monomorphism restriction in ghci to avoid this kind
> of issue.
>
>     :set -XNoMonomoprhismRestriction
>
> which you can put in ~/.ghci so that it is the default for new ghci
> sessions.
>
> --
> brandon s allbery kf8nh                               sine nomine associates
> allbery.b at gmail.com                                  ballbery at sinenomine.net
> unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
>
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