Michael Marte <marte@pms.informatik.uni-muenchen.de> wrote in article <200810232156.19080.marte@pms.informatik.uni-muenchen.de> in gmane.comp.lang.haskell.general:
*FD> :type let x = (1::Int) in x #+ x #+ x let x = (1::Int) in x #+ x #+ x :: (MakeAExp (AExp s) s1) => AExp s1
It appears to me that ghci generates two phantom types s and s1 and fails to unify them.
Because you may define other MakeAExp instances elsewhere, ghc can't unify s and s1. For example, if you were to define instance MakeAExp (FDVar s) [s] ... instance MakeAExp (AExp [s]) s ... then s could be [s1] in your type! You can probably add functional dependencies to fix this problem, but I would suggest that you get rid of MakeAExp altogether. Just give (#+) and its friends the type "AExp s -> AExp s -> AExp s", with no type-class constraint. You need to explicitly inject FDVar into AExp, but most of the time you probably just need to handle AExp values without caring that they are constructed with Variable. You also need to inject Int into AExp, unless you simply make "instance Num (AExp s)". -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig Lemonade was a popular drink and it still is.