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.

For further information, feel free to ask the people responsible for or involved in this proposal:

  • David Luposchainsky aka @quchen, dluposchainsky at the email service of Google
  • Franz Thoma aka @fmthoma
  • Herbert Valerio Riedel aka @hvr


  • 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
  • Estimating the breakage - how much stuff we will break
  • Transitional strategy - how to break as little as possible while transitioning
  • Current status

The problem

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

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 Either, State, IO, and 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 , 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:

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,

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:

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

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

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.

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.

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

[Edward Kmett: We have the contents of the pattern synonym available to us at the definition site. With some work we should be able to expose it enough that the compiler can see through it:

pattern Foo a b = Bar a 0 b
pattern Baz a b c <- Quux a b c

Both of those tell us the "real" desugaring as just another pattern we could recurse into.]


  • What laws should fail follow?
    • Left zero: ∀ s f. fail s >>= f ≡ fail s.
    • Right zero: ∀ v s. v >> fail s ≡ fail s.
  • What is the relationship to MonadPlus?
    • As the laws above indicate, fail is a close relative of mzero. We could suggest a default definition of fail _ = mzero, which shows the intended usage and effect of the MonadFail class.
    • However, we should not remove fail and use only mzero instead.
      • Not all types with Monad instances have MonadPlus instances.
      • Some types do use the String argument to fail. For example, a parser might fail with a message involving positional information. Binary uses fail as their only interface to fail a decoding step.
      • Some types have different definitions for mzero and fail. Although STM is MonadPlus it uses the default fail = error. It should therefore not get a MonadFail instance.
  • Rename fail?
    • No. Old code might use fail explicitly and we should 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)
  • Do we need a class constraint (e.g. Monad) on MonadFail?
    • Yes. The intended use of fail is for desugaring do-notation, not generally for any String -> m a function. Given that goal, we would rather keep the constraints simple as MonadFail m => rather than the somewhat redundant (Monad m, MonadFail m) =>.
  • Can we relax the class constraint from Monad to Applicative?
    • We don't necessarily have to choose now. Since Applicative is a superclass of Monad, it is possible to change the superclass for MonadFail to Applicative later. This will naturally require a migration period, and the name will, of course, become misleading.
    • For the sake of discussion, let's use the following definition:class Applicative f => ApplicativeFail f where fail :: String -> f a
    • Pros
      • ApplicativeDo is coming, and fail may be useful to combine pattern matching and Applicative code.
      • If the Monad constraint is kept, that would force Applicative code with pattern matching to be Monad code.
    • Cons
      • The constraints for Monad code using fail become (Monad m, ApplicativeFail m) => instead of the simpler MonadFail m =>. If we expect the common use of fail to be in Monad — not Applicativedo-notation, this leaves us with more verbose constraints.
    • Here are alternative definitions (with names open to debate) that would allow us to keep the constraints simple:
      • class Applicative f => ApplicativeFail f where failA :: String -> f a
      • class ApplicativeFail m => MonadFail m where fail :: String -> m a; fail = failA
      • Since we do not have much experience using ApplicativeDo, it is not yet clear that this large of a change is useful.
  • Which types with Monad instances will not have MonadFail instances?
    • base: Either
    • transformers:
    • stm: STM
  • What MonadFail instances will be created?
    • base: IO
    • transformers:
      • Proposal for an Either instance using Monad instance in Control.Monad.Trans.Error:instance MonadFail (Either String) where fail = Left

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). The fail provides a forward-compatible MonadFail class for GHC versions prior to GHC 8.0
      import Control.Monad
      -- Control.Monad.Fail import will become redundant in GHC 8.8+
      import qualified Control.Monad.Fail as Fail
      instance Monad Foo where
        (>>=) = <...bind impl...>
        -- NB: `return` defaults to `pure` since GHC 7.10
        -- Monad(fail) will be removed in GHC 8.8+; 
        -- GHC may or may not ignore a definition in terms of MonadFail(fail) (decision pending)
        fail =
      instance Fail.MonadFail Foo where
        fail = < implementation...>
    2. Change your pattern to be irrefutable
    3. Emulate the old behaviour by desugaring the pattern match by hand:
      do Left e <- foobar
      do x <- foobar
         e <- case x of
            Left e' -> e'
            Right r -> error "Pattern match failed" -- Boooo
      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 the warnings. 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. Search for "failable pattern" to find your way to the still pretty raw warnings.

Here are some commands you might find interesting for exploring the logs:

# List all packages generating warnings (57 of them)
grep "is used in the context" ''     | \
    grep -v '(‘\[|Maybe|ReadPrec)'   | \
    perl -pe 's#^(.'')\.log.''$#\1#' | \
    uniq -u

# Histogram of the breaking contexts (mostly IO and parsers)
grep "is used in the context" ''                     | \
    grep -v '(‘\[|Maybe|ReadPrec)'                   | \
    perl -pe 's#^.''in the context ‘([^ ]+).''$#\1#' | \
    sort                                             | \
    uniq -c                                          | \
    sort -rg

Transitional strategy

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

  • GHC 8.0 / 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, clashes with and
    • Add a language extension -XMonadFailDesugaring 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.
    • Add a warning 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, but leave it off by default.
    • Add a warning 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). Leave it off by default.
  • GHC 8.2
    • (nothing happens)
  • GHC 8.4
    • Turn on the warning about missing MonadFail instances that we added in 8.0 by default.
  • GHC 8.6
    • Switch -XMonadFailDesugaring on by default.
    • Warnings are still issued if the desugaring extension has been explicitly disabled.
    • Turn on the warning about explicit definition of fail in Monad that we added in 8.0 by default.
  • GHC 8.8
    • Remove -XMonadFail leaving its effects on at all times.
    • Remove fail from Monad
    • Instead, re-export as and
    • Control.Monad.Fail is now a redundant module that can be considered deprecated.

Current status

  • ZuriHac 2015 (29.5. - 31.5.): 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 language 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 messages are readable, but should be more so. We're still on this.
  • 2015-06-09: Estimated breakage by compiling Stackage. Smaller than expected.
  • 2015-06-09 (late): Published. People seem to like the idea, with a couple of pain points remaining.
  • 2015-06-16: Update 1 posted.
  • 2015-09-18: Patch nearly finished. Some nontrivial tests still fail.
  • 2015-11-17: MFP phase 1 merged.
Last modified 14 months ago Last modified on Jan 21, 2016 5:59:20 PM