Hi, I've been reading Oleg Kiselyov's "Implicit Configurations" paper and while getting the modulus math code to work I've hit a bit of a problem. The code is attached to this message; the problem is in the normalize function:
normalize :: (Modular s a, Integral a) => a -> M s a normalize a = M (mod a (modulus (u :: s)))
Trying this code gives:
$ ghci implicit-config.hs GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( implicit-config.hs, interpreted )
implicit-config.hs:34:24: Could not deduce (Modular s a) from the context (Modular s1 a, Integral a) arising from a use of `modulus' at implicit-config.hs:34:24-38 Possible fix: add (Modular s a) to the context of the type signature for `normalize' In the second argument of `mod', namely `(modulus (u :: s))' In the first argument of `M', namely `(mod a (modulus (u :: s)))' In the expression: M (mod a (modulus (u :: s))) Failed, modules loaded: none. Prelude>
The paper uses the following code, which annotates the return of normalize while at the same time binding the 's' type variable for use on the right hand side:
normalize :: (Modular s a, Integral a) ⇒ a → M s a normalize a :: M s a = M (mod a (modulus (u :: s)))
This code fails with:
$ ghci implicit-config.hs GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( implicit-config.hs, interpreted )
implicit-config.hs:34:0: Parse error in pattern Failed, modules loaded: none. Prelude>
Even with the ScopedTypeVariables extension - am I missing something? I've also tried the actual literate haskell code from the technical report and it fails with the same error (parse error in pattern.) I'm using Mac OS X 10.6 and GHC 6.10.1. Austin