[Haskell-cafe] Does Haskell have this SML syntax?

David Feuer david.feuer at gmail.com
Fri Mar 26 05:37:01 UTC 2021

Let's start with the basics: lambda expressions.

ML says

fn x => blah blah

Haskell spells that

\x -> blah blah

Suppose you want to pattern match on the argument. If you only need one
pattern, that's cool:

\(x,y) -> blah blah

But what if you need more than one pattern? Well, standard ("Report")
Haskell makes you use a case expression:

\mx -> case mx of
    Just x -> blah
    Nothing -> etcetera

But GHC has a widely used language extension to get something more like ML.
If you put

-- The "language" is case insensitive.
-- The LambdaCase is case sensitive.
{-# language LambdaCase #-}

at the very tippy top of your .hs file, or pass -XLambdaCase to GHCi, then
you can write that last one

    Just x -> blah
    Nothing -> etcetera

There has been some discussion of trying to expand that syntax to support
anonymous functions of multiple arguments, but no proposal has been
accepted as yet.

On Fri, Mar 26, 2021, 1:27 AM Galaxy Being <borgauf at gmail.com> wrote:

> I'm sure you've answered my question, but I'm too much of a beginner to
> fathom it. If you could explain, that would be great, but I could also go
> off and try to grok it myself. Again, thanks.
> On Fri, Mar 26, 2021 at 12:09 AM Ignat Insarov <kindaro at gmail.com> wrote:
>> Hello Galaxy Being!
>> You can do this:
>>     module Y where
>>     substitute ∷ (α → Bool) → (α, [α]) → [α]
>>     substitute predicate = \ thing → case thing of
>>       (_, [ ]) → [ ]
>>       (substitution, (x: xs)) →
>>         let remainder = substitute predicate (substitution, xs) in
>>           if predicate x
>>           then substitution: remainder
>>           else x: remainder
>> It is even nicer since we can factor out the common part of the `if`
>> block into a `let … in`. You can also enable the `LambdaCase` language
>> extension and it will let you elide the `thing` thing.
>> I am not sure if this is what your question is really about… In
>> principle, of course Haskell has currying. Actually, functions are
>> usually written in curried form in Haskell. Please let me know if I
>> missed the substance of your question!
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