MonadIO in mtl and transformers

Michael Snoyman michael at
Mon Dec 21 14:41:07 EST 2009

On Mon, Dec 21, 2009 at 12:09 PM, wren ng thornton <
wren at> wrote:

> Antoine Latter wrote:
>> Michael Snoyman wrote:
>>> Is there any reason to export only MonadIO? MonadTrans seems like a good
>>> candidate for that split as well.
>> I guess I've never need the 'lift' function outside of code which used
>> other bits of the mtl, so I did not export it.
>> I'm struggling to come up with a concept which would unify a package
>> containing both the MonadIO class and the MonadTrans class. The mtl
>> made sense, as it contained shed-loads of other classes.
> Maybe it'd be worth just rolling into base, instead of a separate package?
> Both MonadTrans and MonadIO seem as basic to me as MonadPlus. And I don't
> recall there ever being controversy about the design of either class.
> (Moving them to base may mess up folks using new base + old mtl ...but that
> seems fixable by forcing old+old or new+new.)
> --
> Live well,
> ~wren
> By the way, does anyone have an update on where we stand wrt the
mtl/transformers situation?

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