Eike Scholz wrote:
mylength = synAttr listLength
$ *Main> :type synAttr $ synAttr :: (Data b) => ((?stack::[Dyn]) => b -> a) -> Attr a
$ *Main> :type listLength $ listLength :: (?stack::[Dyn]) => List -> Float
$ *Main> :type (synAttr listLength) $ (synAttr listLength) :: Attr Float
$ *Main> :type mylength $ mylength :: (?stack::[Dyn]) => Dyn -> Dyn -> [Dyn] -> Maybe Float
where
type Attr a = Dyn -> Dyn -> [Dyn]-> Maybe a
This may be a bug. But note that both interpretations are legitimate. One way of applying synAttr to listLength is first to apply ?stack to listLength, obtaining listLength' :: List -> Float and creating a (?stack::[Dyn]) constraint on the application node, then specializing listLength' to the type (?stack::a) => List -> Float, then passing that to synAttr. Again, I recommend that you not try to use implicit parameters. -- Ben