Re: [Haskell-cafe] type-class inference

On Fri, Aug 12, 2011 at 19:08, Brandon Allbery wrote:

On Fri, Aug 12, 2011 at 18:52, Patrick Browne wrote:

...

Why does the Haskell :type command only sometimes print the type-class?

Haskell infers the most specific type applicable. If the most specific it can get is a typeclass, that's what it produces; if it can infer an explicit type, it will.

By the way, a possible source of confusion here is the combination of the monomorphism restriction and defaulting, especially GHCi's extended defaulting. The monomorphism restriction says that if you don't provide a way to *easily* infer a type for a binding (in practice this means there are no parameters), Haskell insists that the binding is not polymorphic unless you explicitly provide a type signature. Defaulting is how it accomplishes this: there is a list of default types that can be applied when a concrete type is required and none is available, and the first one that typechecks is used. The Haskell standard specifies Double and Integer as default types; GHCI's extended defaulting (or GHC in general with -XExtendedDefaultRules) adds () aka "unit". So, for example, something that you might expect to be (Num a => a) may end up being Integer due to the monomorphism restriction requiring a concrete type and defaulting supplying one. (There's a widely expressed sentiment that the monomorphism restriction should go away because the confusion it engenders is worse than the problems it solves; on the other hand, GHC recently added a new application of it (monomorphic pattern bindings). In any case, if you want to play around with types in GHCi, you may want to ":set -XNoMonomorphismRestriction -XNoMonoPatBinds" so you can see how types actually behave in the wild.) -- brandon s allbery allbery.b@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms