transformers versus mtl

Ross Paterson ross at
Sat Mar 21 12:40:53 EDT 2009

On Sat, Mar 21, 2009 at 04:19:05PM -0000, Sittampalam, Ganesh wrote:
> Ross Paterson wrote:
> > 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 compatibility layer could continue to export the old separate types
> for State, Writer, etc, couldn't it?

It could, though then they'd be different types than the transformer
versions.  There's a trade-off.

> Are there any problems with adding transformers and monads-fd to
> extralibs pending the Haskell Platform being ready?

That should work, but that wouldn't be till September/October.  I'm not
sure what the HP schedule is.  Another possibility is to upload mtl-2.0
as a compatibility layer.

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