Deprecating fromIntegral

David Feuer david.feuer at gmail.com
Tue Sep 26 19:46:19 UTC 2017


On Tue, Sep 26, 2017 at 2:49 PM, Niklas Hamb├╝chen <mail at nh2.me> wrote:

> It seems that if consensus exists already at a time before, deprecation
> can be planned ahead of time.
> The linked page contains examples of that, e.g. the deprecation of
> `Monad.fail` that is already scheduled for 8.6.

David Luposchainsky drafted the MonadFail proposal in December 2013,
nearly four years ago. But that's not the beginning of the story. In
November 2009, Michael Snoyman wrote, in
https://wiki.haskell.org/index.php?title=Failure&oldid=31465,

> Fail is referring to the "fail" function, which is part of the Monad typeclass and is almost universally despised.

And that's not the beginning of the story either. By March 2008, Real
World Haskell http://book.realworldhaskell.org/read/monads.html
warning about fail. I don't have the history of the text, but it
currently reads, in a giant warning box,

> Beware of fail
>
> Many Monad instances don't override the default implementation of fail that we show here, so in those monads, fail uses error. Calling error is usually highly undesirable, since it throws an exception that callers either cannot catch or will not expect. Even if you know that right now you're executing in a monad that has fail do something more sensible, we still recommend avoiding it. It's far too easy to cause yourself a problem later when you refactor your code and forget that a previously safe use of fail might be dangerous in its new context.

In a comment on the text in March 2008, Lennart Augustssen wrote

> The fail function has no business being in the monad class at all, and it should be removed. So I think you should just discourage people from using it in general.

and the next month Cale Gibbard stated

> [Most] monads can't implement it sensibly, and it's not actually part of the definition of a monad.
>
> The fail function was just a hack to make the do-notation translation as described in the Report "simpler", but it really belongs in a separate typeclass, something more along the lines of the way that Haskell 1.4 did it.

Beginning in February 2008, as
https://stackoverflow.com/questions/8163852/should-i-avoid-using-monad-fail
points out, there was a thread,
http://haskell.1045720.n5.nabble.com/Proposal-Add-Text-Read-maybeRead-Read-a-gt-String-gt-Maybe-a-td3174167.html,
in which several luminaries spoke out, explicitly or implicitly,
against fail being in Monad.

So it seems that there was a fairly clear consensus that having fail
in Monad was a bad idea almost ten years ago, and that it would be
better to put it in a subclass. It's a bit hard to compare that to the
case of fromIntegral, which I think people have generally been only
mildly uncomfortable with, and when we don't know if there is anything
sufficiently better!


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