Re: [Haskell-cafe] upgrading mtl1 to mtl2

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 2/16/11 00:51 , Evan Laforge wrote:

On Tue, Feb 15, 2011 at 7:58 PM, Ivan Lazar Miljenovic wrote:

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On 16 February 2011 14:46, Evan Laforge wrote:

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I just got started on this because packages are starting to use mtl2. I had hoped it would be simple, and hopefully it is, but...

Do I really have to add (Functor m) to the 300 or so functions with (Monad m) on them? Or just not use fmap or applicative?

Use liftM instead of fmap if all of your functions are polymorphic in the type of Monad and you don't want to bother putting in the Functor constraint.

There are hundreds of 'fmap's in there too. And even more <$>s and a scattering of <*>s. And... these functions are convenient! I like using them. If "upgrading" to mtl2 means porting away from Functor and Applicative... then that's even less of a "minor incompatibility"! At least the class context mangling can be mostly done with search and replace.

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Says that the only benefit of 'transformers' is that it's "haskell 98 and thus more portable," but doesn't that come with the caveat that "only if you don't use classes and do all the lifting manually"?

Yes, but some people don't want the extensions.

Ok, so does that mean the migration to transformers so mtl could be deleted is cancelled? Or was rendered obsolete by mtl-2? Or maybe I was imagining it? I think minimal extensions is a worthy goal, but you can hardly position Y as "the next step" for X when all it is is X minus features that people still like...

Yes; as I understand it, mtl2 is transformers + monad-fd, and the standard upgrade path is to go to mtl2. monads-tf is expected to eventually replace monads-fd, but it's not a near future change (for one thing, type families are still being figured out; see the recent thread about injective type functions as an example). It was decided that the mtl2 route would be easier than forcing people to replace mtl with transofrmers + monads-fd. - -- brandon s. allbery [linux,solaris,freebsd,perl] allbery.b@gmail.com system administrator [openafs,heimdal,too many hats] kf8nh -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk1dtWMACgkQIn7hlCsL25WPFwCaA8MbZYbZr/gQ9pa/ttj/E5a0 pTgAoIZ9LMNuq4o7arXmTUZa5qTUFjx4 =NLta -----END PGP SIGNATURE-----