Re: MonadFail proposal (MFP): Moving fail out of Monad

I'm a +1 on this proposal as well. In our private Haskell compiler at work, we have had separate Monad and MonadFail classes since 2010, and it is clearly the more principled way to handle partiality: make it visible in the inferred types. I found that there were very few instances when porting Hackage libraries to our compiler that we came across a need to change type signatures because of MonadFail, and making the change was in all cases easy anyway. Regards, Malcolm On 9 Jun 2015, at 23:19, Edward Kmett wrote:

+1 from me for both the spirit and the substance of this proposal. We've been talking about this in the abstract for a while now (since ICFP 2013 or so) and as concrete plans go, this strikes me as straightforward and implementable.

-Edward

On Tue, Jun 9, 2015 at 10:43 PM, David Luposchainsky wrote: -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1

Hello *,

the subject says it all. After we successfully put `=>` into Monad, it is time to remove something in return: `fail`.

Like with the AMP, I wrote up the proposal in Markdown format on Github, which you can find below as a URL, and in verbatim copy at the end of this email. It provides an overview over the intended outcome, which design decisions we had to take, and how our initial plan for the transition looks like. There are also some issues left open to discussion.

https://github.com/quchen/articles/blob/master/monad_fail.md

Here's a short abstract:

- - Move `fail` from `Monad` into a new class `MonadFail`. - - Code using failable patterns will receive a more restrictive `MonadFail` constraint. Code without this constraint will be safe to use for all Monads. - - Transition will take at least two GHC releases. GHC 7.12 will include the new class, and generate warnings asking users to make their failable patterns compliant. - - Stackage showed an upper bound of less than 500 breaking code fragments when compiled with the new desugaring.

For more details, refer to the link or the paste at the end.

Let's get going!

David aka quchen

=============================================================== =============================================================== ===============================================================

`MonadFail` proposal (MFP) ==========================

A couple of years ago, we proposed to make `Applicative` a superclass of `Monad`, which successfully killed the single most ugly thing in Haskell as of GHC 7.10.

Now, it's time to tackle the other major issue with `Monad`: `fail` being a part of it.

You can contact me as usual via IRC/Freenode as *quchen*, or by email to *dluposchainsky at the email service of Google*. This file will also be posted on the ghc-devs@ and libraries@ mailing lists, as well as on Reddit.

Overview - --------

- - **The problem** - reason for the proposal - - **MonadFail class** - the solution - - **Discussion** - explaining our design choices - - **Adapting old code** - how to prepare current code to transition smoothly - - **Esimating the breakage** - how much stuff we will break (spoiler: not much) - - **Transitional strategy** - how to break as little as possible while transitioning - - **Current status**

The problem - -----------

Currently, the `<-` symbol is unconditionally desugared as follows:

```haskell do pat <- computation >>> let f pat = more more >>> f _ = fail "..." >>> in computation >>= f ```

The problem with this is that `fail` cannot (!) be sensibly implemented for many monads, for example `State`, `IO`, `Reader`. In those cases it defaults to `error`. As a consequence, in current Haskell, you can not use `Monad`-polymorphic code safely, because although it claims to work for all `Monad`s, it might just crash on you. This kind of implicit non-totality baked into the class is *terrible*.

The goal of this proposal is adding the `fail` only when necessary and reflecting that in the type signature of the `do` block, so that it can be used safely, and more importantly, is guaranteed not to be used if the type signature does not say so.

`MonadFail` class - -----------------

To fix this, introduce a new typeclass:

```haskell class Monad m => MonadFail m where fail :: String -> m a ```

Desugaring can now be changed to produce this constraint when necessary. For this, we have to decide when a pattern match can not fail; if this is the case, we can omit inserting the `fail` call.

The most trivial examples of unfailable patterns are of course those that match anywhere unconditionally,

```haskell do x <- action >>> let f x = more more >>> in action >>= f ```

In particular, the programmer can assert any pattern be unfailable by making it irrefutable using a prefix tilde:

```haskell do ~pat <- action >>> let f ~pat = more more >>> in action >>= f ```

A class of patterns that are conditionally failable are `newtype`s, and single constructor `data` types, which are unfailable by themselves, but may fail if matching on their fields is done with failable paterns.

```haskell data Newtype a = Newtype a

- -- "x" cannot fail do Newtype x <- action >>> let f (Newtype x) = more more >>> in action >>= f

- -- "Just x" can fail do Newtype (Just x) <- action >>> let f (Newtype (Just x)) = more more >>> f _ = fail "..." >>> in action >>= f ```

`ViewPatterns` are as failable as the pattern the view is matched against. Patterns like `(Just -> Just x)` should generate a `MonadFail` constraint even when it's "obvious" from the view's implementation that the pattern will always match. From an implementor's perspective, this means that only types (and their constructors) have to be looked at, not arbitrary values (like functions), which is impossible to do statically in general.

```haskell do (view -> pat) <- action >>> let f (view -> pat) = more more >>> f _ = fail "..." >>> in action >>= f

do (view -> ~pat) <- action >>> let f (view -> ~pat) = more more >>> in action >>= f ```

A similar issue arises for `PatternSynonyms`, which we cannot inspect during compilation sufficiently. A pattern synonym will therefore always be considered failable.

```haskell do PatternSynonym x <- action >>> let f PatternSynonym x = more more >>> in f _ = fail "..." >>> in action >>= f ```

Discussion - ----------

- - Although for many `MonadPlus` `fail _ = mzero`, a separate `MonadFail` class should be created instead of just using that.

- A parser might fail with an error message involving positional information. Some libraries, like `Binary`, provide `fail` as their only interface to fail a decoding step.

- Although `STM` is `MonadPlus`, it uses the default `fail = error`. It will therefore not get a `MonadFail` instance.

- - What laws should `fail` follow? **Left zero**,

```haskell ∀ s f. fail s >>= f ≡ fail s ```

A call to `fail` should abort the computation. In this sense, `fail` would become a close relative of `mzero`. It would work well with the common definition `fail _ = mzero`, and give a simple guideline to the intended usage and effect of the `MonadFail` class.

- - Rename `fail`? **No.** Old code might use `fail` explicitly and we might avoid breaking it, the Report talks about `fail`, and we have a solid migration strategy that does not require a renaming.

- - Remove the `String` argument? **No.** The `String` might help error reporting and debugging. `String` may be ugly, but it's the de facto standard for simple text in GHC. No high performance string operations are to be expected with `fail`, so this breaking change would in no way be justified. Also note that explicit `fail` calls would break if we removed the argument.

- - How sensitive would existing code be to subtle changes in the strictness behaviour of `do` notation pattern matching? **It doesn't.** The implementation does not affect strictness at all, only the desugaring step. Care must be taken when fixing warnings by making patterns irrefutable using `~`, as that *does* affect strictness. (Cf. difference between lazy/strict State)

- - The `Monad` constraint for `MonadFail` seems unnecessary. Should we drop or relax it? What other things should be considered?

- Applicative `do` notation is coming sooner or later, `fail` might be useful in this more general scenario. Due to the AMP, it is trivial to change the `MonadFail` superclass to `Applicative` later. (The name will be a bit misleading, but it's a very small price to pay.) - The class might be misused for a strange pointed type if left without any constraint. This is not the intended use at all.

I think we should keep the `Monad` superclass for three main reasons:

- We don't want to see `(Monad m, MonadFail m) =>` all over the place. - The primary intended use of `fail` is for desugaring do-notation anyway. - Retroactively removing superclasses is easy, but adding them is hard (see AMP).

Adapting old code - -----------------

- - Help! My code is broken because of a missing `MonadFail` instance!

*Here are your options:*

1. Write a `MonadFail` instance (and bring it into scope)

```haskell #if !MIN_VERSION_base(4,11,0) -- Control.Monad.Fail import will become redundant in GHC 7.16+ import qualified Control.Monad.Fail as Fail #endif import Control.Monad

instance Monad Foo where (>>=) = <...bind impl...> -- NB: `return` defaults to `pure`

#if !MIN_VERSION_base(4,11,0) -- Monad(fail) will be removed in GHC 7.16+ fail = Fail.fail #endif

instance MonadFail Foo where fail = <...fail implementation...> ```

2. Change your pattern to be irrefutable

3. Emulate the old behaviour by desugaring the pattern match by hand:

```haskell do Left e <- foobar stuff ```

becomes

```haskell do x <- foobar e <- case foobar of Left e' -> e' Right r -> error "Pattern match failed" -- Boooo stuff ```

The point is you'll have to do your dirty laundry yourself now if you have a value that *you* know will always match, and if you don't handle the other patterns you'll get incompleteness warnings, and the compiler won't silently eat those for you.

- - Help! My code is broken because you removed `fail` from `Monad`, but my class defines it!

*Delete that part of the instance definition.*

Esimating the breakage - ----------------------

Using our initial implementation, I compiled stackage-nightly, and grepped the logs for found "invalid use of fail desugaring". Assuming my implementation is correct, the number of "missing `MonadFail`" warnings generated is 487. Note that I filtered out `[]`, `Maybe` and `ReadPrec`, since those can be given a `MonadFail` instance from within GHC, and no breakage is expected from them.

The build logs can be found [here][stackage-logs]. Search for "failable pattern" to find your way to the still pretty raw warnings.

Transitional strategy - ---------------------

The roadmap is similar to the [AMP][amp], the main difference being that since `MonadFail` does not exist yet, we have to introduce new functionality and then switch to it.

* **GHC 7.12 / base-4.9**

- Add module `Control.Monad.Fail` with new class `MonadFail(fail)` so people can start writing instances for it.

`Control.Monad` only re-exports the class `MonadFail`, but not its `fail` method.

NB: At this point, `Control.Monad.Fail.fail` clashes with `Prelude.fail` and `Control.Monad.fail`.

- *(non-essential)* Add a language extension `-XMonadFail` that changes desugaring to use `MonadFail(fail)` instead of `Monad(fail)`.

This has the effect that typechecking will infer a `MonadFail` constraint for `do` blocks with failable patterns, just as it is planned to do when the entire thing is done.

- Warn when a `do`-block that contains a failable pattern is desugared, but there is no `MonadFail`-instance in scope: "Please add the instance or change your pattern matching." Add a flag to control whether this warning appears.

- Warn when an instance implements the `fail` function (or when `fail` is imported as a method of `Monad`), as it will be removed from the `Monad` class in the future. (See also [GHC #10071][trac-10071])

3. GHC 7.14

- Switch `-XMonadFail` on by default. - Remove the desugaring warnings.

3. GHC 7.16

- Remove `-XMonadFail`, leaving its effects on at all times. - Remove `fail` from `Monad`. - Instead, re-export `Control.Monad.Fail.fail` as `Prelude.fail` and `Control.Monad.fail`. - `Control.Monad.Fail` is now a redundant module that can be considered deprecated.

Current status - --------------

- - [ZuriHac 2015 (29.5. - 31.5.)][zurihac]: Franz Thoma (@fmthoma) and me (David Luposchainsky aka @quchen) started implementing the MFP in GHC.

- Desugaring to the new `fail` can be controlled via a new langauge extension, `MonadFailDesugaring`. - If the language extension is turned off, a warning will be emitted for code that would break if it was enabled. - Warnings are emitted for types that *have* a *MonadFail* instance. This still needs to be fixed. - The error message are readable, but should be more so. We're still on this. - - 2015-06-09: Estimated breakage by compiling Stackage. Smaller than expected.

[amp]: https://github.com/quchen/articles/blob/master/applicative_monad.md [stackage-logs]: https://www.dropbox.com/s/knz0i979skam4zs/stackage-build.tar.xz?dl=0 [trac-10071]: https://ghc.haskell.org/trac/ghc/ticket/10071 [zurihac]: https://wiki.haskell.org/ZuriHac2015

-----BEGIN PGP SIGNATURE----- Version: GnuPG v1

iQEcBAEBAgAGBQJVd0/yAAoJELrQsaT5WQUshbUH/A3W0itVAk7ao8rtxId5unCJ 7StriKVkTyLAkkrbRJngM4MHEKiCsoyIgr8kBIwSHgk194GxeP2NCF4ijuBZoDBt +Uci+6BCBinV8+OzfrfTcJb4+8iw1j+eLWJ/Nz/JDMDNCiyzyC0SMsqGa+ssOz7H /2mqPkQjQgpHuP5PTRLHKPPIsayCQvTbZR1f14KhuMN2SPDE+WY4rqugu//XuIkN u1YssIf5l8mEez/1ljaqGL55cTI0UNg2z0iA0bFl/ajHaeQ6mc5BAevWfSohAMW7 7PIt13p9NIaMHnikmI+YJszm2IEaXuv47mGgbyDV//nHq3fwWN+naB+1mPX2eSU= =vPAL -----END PGP SIGNATURE----- _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries