ListT considered harmful

Fumiaki Kinoshita

27 Mar 2015 27 Mar '15

12:03 a.m.

It is well known that ListT m in transformers is not a monad unless the underlying monad is commutative. The fact that ListT is not a monad transformer is sufficient to remove it from the package, and several packages already defined their own ListT. Oughtn't we deprecate ListT or replace it by a correct one?

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Michael Snoyman

27 Mar 27 Mar

12:07 a.m.

I'm in favor of deprecation, but opposed to removing any time soon due to backwards compatibility concerns. On Fri, Mar 27, 2015 at 7:03 AM Fumiaki Kinoshita wrote:

...

It is well known that ListT m in transformers is not a monad unless the underlying monad is commutative. The fact that ListT is not a monad transformer is sufficient to remove it from the package, and several packages already defined their own ListT.

Oughtn't we deprecate ListT or replace it by a correct one? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

Chris Wong

1:36 a.m.

On Fri, Mar 27, 2015 at 5:07 PM, Michael Snoyman wrote:

...

I'm in favor of deprecation, but opposed to removing any time soon due to backwards compatibility concerns.

I agree. Given that we just deprecated ErrorT (which is both more popular, and does follow the monad laws) I think there is good reason to phase out transformers' ListT as well. I've attached a patch that adds the relevant pragma. I'm not familiar with darcs so excuse me if it's messed up :) libraries list and/or Ross Paterson, do you have anything to say about this issue?

...

On Fri, Mar 27, 2015 at 7:03 AM Fumiaki Kinoshita wrote:

...
It is well known that ListT m in transformers is not a monad unless the underlying monad is commutative. The fact that ListT is not a monad transformer is sufficient to remove it from the package, and several packages already defined their own ListT.

Oughtn't we deprecate ListT or replace it by a correct one? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

-- Chris Wong https://lambda.xyz

Ross Paterson

6:23 a.m.

On Fri, Mar 27, 2015 at 01:03:04PM +0900, Fumiaki Kinoshita wrote:

...

It is well known that ListT m in transformers is not a monad unless the underlying monad is commutative. The fact that ListT is not a monad transformer is sufficient to remove it from the package, and several packages already defined their own ListT.

Oughtn't we deprecate ListT or replace it by a correct one?

This flaw is at least documented, and it does severely limit the usefulness of ListT. Deprecation seems reasonable, but we'd need to offer a replacement first.

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participants (4)

Chris Wong
Fumiaki Kinoshita
Michael Snoyman
Ross Paterson