Hi,
In Chapter 6 of "Beginning Haskell" by Apress there's a couple of classes introduced for vectors and things that can be vectorized (not related to Data.Vector) {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} class Vector v where distance :: v -> v -> Double instance Vector (Double, Double) where distance (a,b) (c,d) = sqrt $ (c - a) * (c - a) + (d - b) * (d - b) class Vectorizable e v where toVector :: e -> v instance Vectorizable (Double, Double) (Double, Double) where toVector = id x = 1.0 :: Double y = 10.0 :: Double While I understand how to use the "distance" function: ghci> distance (x, x) (y, y) 12.727922061357855 ... and I can see how "toVector" is used in their code 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? Derek. |
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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20140310/779b510e/attachment.html> |
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 > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > |
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