To the best of my knowledge, `InstanceSigs` are never strictly necessary. They can, however, be useful for at least four purposes:
1. To provide a compiler-checked reminder of the type. 2. To bind type variables with `ScopedTypeVariables`. 3. To generalize the type so you can use polymorphic recursion. 4. To enhance parametricitry/polymorphism for internal documentation purposes. The third reason is probably the main technical one to allow a more general signature, but the fourth is likely helpful too. On Sun, Aug 8, 2021, 3:04 AM Anthony Clayden <anthony.d.clay...@gmail.com> wrote: > I can't help but feel InstanceSigs are either superfluous or upside-down. > It's this bit in the User Guide: > > > The type signature in the instance declaration must be > > more polymorphic than (or the same as) the one in the class declaration, > > instantiated with the instance type. > > Usually if you give a signature, it must be _less_ polymorphic (or the > same as) the type inferred from the term: > > > lessPolyPlus :: Integral a => a -> a -> a > > lessPolyPlus x y = x + y > > Or > > > lessPolyPlus (x :: a) y = x + y :: Integral a => a > > The examples in the User Guide aren't helping: you could just drop the > InstanceSigs, and all is well-typed. (Even the example alleging to use > -XScopedTypeVariables in a where sub-decl: you could just put random `xs :: > [b]` without scoping `b`.) > > Dropping the Sigs altogether works because the type from the class decl, > suitably instantiated, is less polymorphic than inferred from the term. IOW > the suitably instantiated type restricts what would otherwise be inferred. > Situation normal. > > I suppose it might be helpful to give an explicit InstanceSig as 'belt and > braces' for the instantiated -- possibly because the instantiation is hard > to figure out; possibly because you want to use -XScopedTypeVariables > within a where-bound sub-decl, as an extra piece of string. > > I can see you mustn't make the InstanceSig _less_ polymorphic than the > suitably instantiated. > > But the docos don't give any example where it's essential to provide an > InstanceSig _and_ make it strictly more polymorphic. Here all the sigs and > annotations are just superfluous: > > > maxPolyPlus :: Num a => a -> a -> a > > maxPolyPlus = (+) > > > > class C a where foo :: a -> T a > > instance Integral a => C a where > > foo :: Num a => a -> T a > > foo (x :: a) = MkT (maxPolyPlus x x :: Num a => a) > > Is there a persuasive example (to put in the User Guide)? > > AntC > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users >
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