Martin Huschenbett wrote:
readValue :: Field -> (forall s. SqlBind s => s) -> Value readValue _ = ...
That works just fine. But now I want a version of readValue that has a Maybe wrapped around the second parameter and that shall call readValue in the case of a Just and emptyValue in the case of Nothing. But I can't figure out how to write this function as I always get compiler errors.
-- The type I want to get. readValue' :: Field -> (forall s. SqlBind s => Maybe s) -> Value [..] -- Fourth trial: readValue fld s = case s of Just s' -> readValue fld s' Nothing -> emptyValue fld
For me, the fourth trial works, at least on f :: (forall s . Num s => Maybe s) -> Int f y = case y of Just x -> x Nothing -> 0 Your definition has a typo (missing tick '), maybe that's the cause why it doesn't work? Furthermore, I guess that let-binding any troublesome rank-2-variables is beneficial. Regards, apfelmus