[Haskell-cafe] Monad transformer responsibilities

[Ryan Ingram]

>From what I understand, the current best practices are to build your
package dependencies like so:

Parsec    MyMonadT
 MyMonadT_Parsec   -- orphan instances go here
 ProjectPackage

This does mean splitting up your project into three packages, but
decouples the orphan instance into its own package where it can do the
least damage :)

At the very least it should go into its own module which can be
imported only in the places that need it, similar to
Control.Monad.Instances defining the orphan instance for Monad ((->)
r).

  -- ryan

On Fri, Jun 5, 2009 at 2:54 AM, Martijn van
Steenbergen<martijn at van.steenbergen.nl> wrote:
> Hello,
>
> Suppose I have two projects: 1) one that defines a monad transformer and an
> accompanying type class that captures my monad-specific operations and 2)
> one that uses the other project, combining the monad transformer with, say,
> Parsec.
>
> Now while writing my Parsec parser I want to use my monad transformer
> operations without using lift: I need an instance MyMonadT Parsec. Where
> should this instance go? I can think of three answers, all unsatisfactory:
>
> 1) For obvious reasons it shouldn't go in the Parsec package.
>
> 2) For pretty much the same reasons it shouldn't go in my monad transformer
> package, either. Also, it is undesirable to add a dependency on Parsec just
> for this instance, and the package should not have to know about the
> projects that are going to use it.
>
> 3) If I put it in the second project it is an orphan instance, which is
> undesirable for well-known reasons.
>
> What is the best solution?
>
> Thank you,
>
> Martijn.
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