Question

I have a data type :

data BuildException a = KillBuild JobID a Stage
                      | FailBuild JobID a Stage
                      | CancelBuild JobID a Stage
                      | StopBuild JobID a Stage
                         deriving Typeable

where a has to have an instance for class Foo. I remember reading (in RWH, maybe) that
while it was possible to have class constraints in a data definition, it was not desirable.
What’s the proper way to do this then?

Answer 1

Simply apply the type constraints on functions that need it. For example:

makeException :: Foo a => String -> BuildException a

There might even be some functions that don’t need the Foo constraint, which might make it possible for the client of your API to use a subset of it without having to define the Foo constraint! That wouldn’t be possible if it was part of the data record definition.