Hi Dominic,
On Dec 15, 2007 10:38 AM, Dominic Steinitz
Supply.hs:33:13: Couldn't match expected type `g' against inferred type `MyGen' `g' is a rigid type variable bound by the type signature for `randomR' at <no location info> Expected type: (Foo, Foo) -> g -> (Foo, g) Inferred type: (Foo, Foo) -> MyGen -> (Foo, MyGen) In the expression: myRandomR In the definition of `randomR': randomR = myRandomR Failed, modules loaded: none.
I have two questions:
1. Why can't I instantiate a type class with any function I like provided it fits the type signature?
The problem here is that myRandomR does not match the type signature in the class. You have myRandomR :: (Foo, Foo) -> MyGen -> (Foo, MyGen) but you need randomR :: RandomGen g => (Foo, Foo) -> g -> (Foo, g) i.e., the function is required to be more generic than the one you provide. The good news is that you don't need to declare Foo at all. You only need to declare an instance of Random if you want to generate random *values* of some new type. If you just want numbers, remove all the stuff about Foo and call (for example) randomR (0,5) $ MyGen 18 and it'll give you a number and a new MyGen.
2. Is this the right approach to generating predictable arbitrary values? Are there others?
Seems entirely fine to me. Hope this helps, - Benja