[Haskell-cafe] bizarre syntax error

Geoffrey Irving irving at naml.us
Sat Jun 27 20:37:00 EDT 2009

I ran into a unfortunate syntax error just now.  I figured I'd share
it because it's the weirdest message I've ever gotten out of ghc.  The
broken code is

    case t of
      TyApply tv types -> do
        (tvl, cases) <- lookupDatatype prog tv
        let tenv = Map.fromList (zip tvl types)  -- GHC POINTS HERE
            caseType (c,vl,e')
              | Just tl <- List.lookup c cases =
                  if length vl == a then
                    expr prog global (foldl (\e (v,t) -> Map.insert v
t e) env (zip vl (map (subst tenv) tl))) e'
                    typeError ("arity mismatch in pattern: "++show
(pretty c)++" expected "++show a++" argument"++(if a == 1 then "" else
                      ++" but got ["++concat (intersperse ", " (map
(show . pretty) vl))++"]")
                  where a = length tl -- THIS IS THE PROBLEM
              | otherwise = typeError ("datatype "++show (pretty
tv)++" has no constructor "++show (pretty c))
            defaultType Nothing = return []
            defaultType (Just (v,e')) = expr prog global (Map.insert v
t env) e' >>=. \t -> [t]
            join t1 t2 | Just t <- unifyS t1 t2 = return t
                       | otherwise = typeError ("failed to unify types
"++show (pretty t1)++" and "++show (pretty t2)++" from different case
        caseResults <- mapM caseType pl
        defaultResults <- defaultType def
        foldM1 join (caseResults ++ defaultResults)
      _ -> typeError ("expected datatype, got "++show (pretty t))

ghc complains that

    "The last statement in a 'do' construct must be an expression"

and points to the line declaring "tenv".  The actual problem is that
you can't put a "where" block in between two pattern guards. :)


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