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Nitpicks

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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.

Contents

1 Syntax-related nitpicks

example = do
   ╎params <- loadParams
    let╎request = buildRequest params
            & fixRequest
   ╎response <- remoteCall request
    let╎Just theValue = responseValueMay response
   ╎return theValue
example = do
   ╎params <- loadParams
   ╎request = buildRequest params
        & fixRequest
   ╎response <- remoteCall request
   ╎Just theValue = responseValueMay response
   ╎return theValue
See https://mail.haskell.org/pipermail/haskell-cafe/2012-August/102741.html
  • 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)
See https://mail.haskell.org/pipermail/haskell-cafe/2014-May/114287.html

2 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 - http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/117846

3 Semantics-related nitpicks

4 Base-related nitpicks