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This page is for people to record nitpicks about the Haskell language.

A "nitpick", in this case, is something that is annoying or could be improved, but is probably not important enough to justify the added complexity of tacking it on as an extension or breaking existing code.

In other words, if we could go back in time and fix it before it happened, we probably would, but now it would probably be too onerous.

Ideally, these nitpicks could help to inform future proposals or compatibility-breaking changes to the language. Even if they may be too onerous to change right now, it's possible that it would make sense to address them at some other time.

If the nitpick has been discussed at length, please post a link to the discussion.

Syntax-related nitpicks

example = do
   params <- loadParams
    letrequest = buildRequest params
            & fixRequest
   response <- remoteCall request
    letJust theValue = responseValueMay response
   return theValue
example = do
   params <- loadParams
   request = buildRequest params
        & fixRequest
   response <- remoteCall request
   Just theValue = responseValueMay response
   return theValue
  • Add "monad extraction" operator (I used a "!" one, because it's present in Idris ). Often you don't care in which order monad values are "extracted", and you just want to use their values in parameters to function-call or return. Compare:
  • "do" syntax
do  params <- loadParams
    time <- getCurrentTime
    user <- getCurrentUser
    getExampleData params time user
  • monad extraction
getExampleData !loadParams !getCurrentTime !getCurrentUser
  • Applicative lift
bind3 getExampleData loadParams getCurrentTime getCurrentUser
where bind3 f x y z = join (liftA3 f x y z)
  • Change do-notation to more closely resemble list-comprehensions:
do info | params <- loadParams,
          time <- getCurrentTime,
          user <- getCurrentUser,
          info <- getExampleData params time user
See Use more existing sugar in Do notation considered harmful.

Syntactic-sugar related nitpicks

  • It is not possible to create non-recursive bindings in do-blocks. Some syntactic sugar, say, an "assignment arrow" foo <-= modify foo which desugars to foo' (modify foo) where foo' foo = ..., would solve this problem, and can be used instead of let. The primary motivation for this is that it is currently not possible to "mutate" bindings in do-blocks, for example - let foo = modify foo would be interpreted as a recursive definition instead. So we have to invent new variable names to refer to the mutated values (suffixing (') being the most common), and since the old binding is still in scope there is no way to ensure that the old value will not be accidentally used, causing bugs. A universal non-recursive let would also solve this problem but it has its own issues, and is a much bigger change to the language. Some relevant discussion here -

Semantics-related nitpicks

Base-related nitpicks

  • ...or just drop type classes altogether and use e.g. System CT : for top-level definitions by default and use annotations elsewhere, like polymorphic let-expressions :. An account of a more modest implementation is also available (see chapter 7):.
  • Data.List.nub and other "Set" operations should be restricted to Ord not Eq in order to reduce their complexity. It is very unlikely for anyone to create a datatype that can support Eq but not Ord. Data.Set in the "container" package assumes Ord as well.
  • Partial functions like head and tail in Prelude. The problem is in their partiality.
  • The proliferation of questions, guides, tutorials, lessons, introductions et al about the topic indicates that I/O being both abstract and monadic is at best pedagogically dubious. Instead, I/O should be defined more directly in standard Haskell:
     -- this is merely a suggestion...
    type IO# a = (a -> IOCall#) -> IOCall#
    data IOCall# -- abstract
which allows beginners to acquire a better understanding of how I/O works - the monadic abstraction can then be taught later in an intermediate or advanced course.