2012/1/3 Yucheng Zhang
(Hopefully being a little more explicit about this can help you understand where things are going wrong.) [--snip--]
legSome :: LegGram nt t s -> nt -> Either String ([t], s)
The 'nt' you see above
legSome (LegGram g) ntV = case Main.lookup ntV g of Nothing -> Left "No word accepted!" Just l -> let sg = legSome (LegGram (Main.delete ntV g)) subsome :: [RRule nt t s] -> Either String ([t], s)
... isn't the same as the 'nt' you see in this line, so it constrains 'subsome' to a different type than the one you intended -- and indeed one which can't be unified with the inferred type. (Unless you use ScopedTypeVariables.) As Brent suggested, you should probably pull the "subsome" function out into a top-level function in any case. Cheers,