[Haskell-beginners] understanding type classes and class constraints

[Robert Krahn]

Thanks a lot for the thorough and quick explanations!


On Sun, Nov 17, 2013 at 6:57 PM, Brandon Allbery <allbery.b at gmail.com>wrote:

> On Sun, Nov 17, 2013 at 7:47 PM, Robert Krahn <robert.krahn at gmail.com>wrote:
>
>> instance Tester Integer where
>>     test 0 = False
>>     test _ = True
>>
>> For the Maybe instance I want to delegate to the value of Just and add a
>> class constraint:
>>
>> instance (Tester m) => Tester (Maybe m) where
>>     test Nothing = False
>>     test (Just x) = test x
>>
>> It compiles nicely and works for Just values
>> test (Just 3) -- True
>> test (Just 0) -- False
>>
>> But
>> test Nothing
>>
>> gives me
>>     No instance for (Tester a0) arising from a use of `test'
>>
>
> Several things are happening here. First is that numbers undergo
> defaulting; with nothing else to specify the type of `3` or `0`, they
> default to Integer and all is well.
>
> This cannot be said of `Nothing`; it needs to pick an instance, but
> `Nothing` doesn't give it anything to work from.
>
> "But I only have an instance for Integer!" Typeclasses work under the
> "open world assumption": a typeclass could be added at any time, so it
> cannot commit to a given typeclass instance simply because it only happens
> to know of one valid instance. In particular, it cannot conclude that
> `Nothing` is of type `Maybe Integer` solely because it happens to only be
> aware of a `Tester Integer` instance.
>
> (Instances are program global and cannot be hidden, so not making the open
> world assumption could easily cause programs to completely change their
> meaning with the addition of a single additional instance *anywhere* in
> that program, even if not imported to that module.)
>
> --
> 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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