On Fri, Mar 20, 2009 at 9:47 AM, Ross Paterson
On Thu, Mar 19, 2009 at 05:14:22PM -0000, Sittampalam, Ganesh wrote:
Would now be a good time to start it, or do we need more experience of transformers?
Perhaps sooner would be better that later, as there are so many packages involved (~350, of which over 100 include mtl types and classes in their interfaces).
I think it's necessary to split off the multi-parameter type classes from the rest, so that the transformers can be used either on their own or with type families, and clients of the different configurations can be used together.
A transition will be disruptive, as only 30 of the packages have upper bounds on their mtl dependencies, but I think we can reduce the disruption.
The combination transformers + monads-fd is close to a compatible replacement for mtl at the source level, except that State, Writer, etc are now type synonyms, which will break some code. Of course the package names are also different. We can't just rename monads-fd as mtl-2.0, because Control.Monad.Identity and Control.Monad.Trans are in the transformers package. But we could make a compatibility version of mtl along the lines of base3-compat. (And we need to rename the modules in monads-tf.)
The other thing to think about is with regards to FunctionalDependencies vs. TypeFamilies. If we have two different libraries, one providing a TF interface and one providing an FD interface, we will naturally have some developers using one of them and others using another, based on personal preference. This is all well and good for application code, but it gets messy for libraries. If package foo declares some datatypes and makes them instances of monads-tf classes, and package bar declares some datatypes and makes them instances of monads-fd classes, then it will be hard for an application or another library to use both foo and bar in conjunction with each other because they will not be using the same monad classes. Would it be best practice for libraries to depend on *both* packages and declare instances for both classes? Alex