Having worked around the other bug while Malcolm tries to figure it out (sorry Malcolm, didn't mean to land such a big chunk of work on your lap), I think I've hit another hat-trans bug. The following code: lookupAlias :: AliasTable -> Type -> Type lookupAlias (AliasTable table) (TLit x) = typ where (0, typ) = Map.findWithDefault (0, TLit x) x table lookupAlias a@(AliasTable table) (TList (TLit x:xs)) = if isJust res then some else none where res = Map.lookup x table none = TList (TLit x : map (lookupAlias a) xs) Just (n, typ) = res some = if n == length xs then mapNumber (xs !!) typ else error "lookupAlias: mismatch" lookupAlias _ x = x Gets translated to: ... lots of stuff ... hlookupAlias (T.R (AliasTable ftable) _) (T.R (TLit fx) _) p = gtyp p58v43v58v45 p where gtyp ptyp p = T.constUse ptyp p styp j59v11v59v18typ = case T.cguard p59v11v59v18 p 148 > (T.ap2 p59v12v59v12 p (p59v12v59v12 Hat.Prelude.!== p) fv59v12v59v12n (T.ap1 p59v12v59v12 p (Hat.PreludeBasic.gfromInteger p59v12v59v12 p) 150 > (T.conInteger p59v12v59v12 p 0))) (\ p -> h ftyp p) (\ p -> T.fatal p) of T.R (T.Tuple2 fv59v12v59v12n ftyp) ktyp -> (ktyp,ftyp) _ -> T.fatal p where h ftyp p = T.app3 p59v22v59v60 p59v22v59v40 p Hat.Map.afindWithDefault Hat.Map.hfindWithDefault (T.con2 p59v42v59v52 p T.Tuple2 T.aTuple2 (T.ap1 p59v43v59v43 p (Hat.PreludeBasic.gfromInteger p59v43v59v43 p) (T.conInteger p59v43v59v43 p 0)) (T.con1 p59v46v59v51 p TLit aTLit fx)) fx ftable h _ p = T.fatal p ... lots of stuff... Note in the case statement that the variable fv59v12v59v12n is bound in the second case, but used in the first... As is the variable ftyp. This of course leads to these errors: Hat/TypeAlias.hs:148:63: Not in scope: `fv59v12v59v12n' Hat/TypeAlias.hs:150:56: Not in scope: `ftyp' Bob