I agree with Michal. Additionally, I'm not clear how `const/1` could be used in Bruce's example at all.
To elaborate, `fn -> foo() end` and `const(foo())` cannot be equivalent when `const/1` is merely a function. This becomes readily apparent when `foo()` is side effects or side causes. In the first case, `foo()` is never evaluated until the wrapping function is called, in the case of `const/1` however the function is evaluated and then its return value bound over by a function. Eg: `fn -> DateTime.utc_now() end` will always return the current time when evaluated, where as `const(DateTime.utc_now())` will evaluate the current time once and then always return that same time. That might sound useful, except that we already can do that by simply binding the return value of `DateTime.utc_now()` to a variable and passing that variable around. I'm having difficulty coming up with a scenario where, instead of simply having the value, I have the value wrapped in an anonymous function that I need to call. Consequently, I struggle to see where `const/1` can actually be used, or how it would work. In the example in the initial proposal, there is this: ``` Enum.map([0,1,2,3], &Function.const/2) ``` As a minor note, presumably that should be `const/1`, right? More importantly, what is the return value here? If it's `[0, 1, 2, 3]` then `const/1` is equivalent to `identity`. If it's ``` [fn -> 1 end, fn -> 2 end, fn -> 3 end, fn -> 4 end] ``` then a simple list of integers seems universally more useful. On Tuesday, February 4, 2020 at 5:03:39 AM UTC-5, Michał Muskała wrote: > > I’d argue back that this particular pattern, where you want a list of > fixed length with the same value, is much better served by > `List.duplicate/2`. > > > > I think in general, higher order combinator functions like identity, > const, flip, and friends are usually used to facilitate the point-free > style of programming in languages like Haskell. And in general point-free > style usually does not lead to the most readable code. > > > > Again, referring to the example provided, if I know anonymous functions, I > know what’s going on. When using `Funcion.const`, I have to understand that > concept as well. There’s one extra thing to learn. > > > > Michał. > > > > *From: *"elixir-l...@googlegroups.com <javascript:>" < > elixir-l...@googlegroups.com <javascript:>> on behalf of Bruce Tate < > br...@grox.io <javascript:>> > *Reply to: *"elixir-l...@googlegroups.com <javascript:>" < > elixir-l...@googlegroups.com <javascript:>> > *Date: *Monday, 3 February 2020 at 11:47 > *To: *"elixir-l...@googlegroups.com <javascript:>" < > elixir-l...@googlegroups.com <javascript:>> > *Subject: *Re: [elixir-core:9353] [Proposal] Add Function.const/2 > > > > My counterpoint is this. Any time you can name a concept that makes it > easier to see what's going on, it's important. > > > > Examples: > > * Create back padding of four blank table cells > > * use with Stream.repeatedly and take, for example, to initialize a > data structure for OTP. > > > > I do these two things with pretty good frequency because I build > responsive layouts often needing tabular structure, but without HTML > tables. > > > > Say you are laying out tables that are responsive but without HTML tables. > You > > d want to add padding to the end of uneven rows. To create the padding > you'd do > > > > Stream.repeatedly( fn -> :padding end) |> Enum.take(4) > > > > where :padding is the constant padding you want. This pattern comes up > with some regularity in my user interfaces. It doesn't hurt anything, and > it would be a great addition to the function module. > > > > I like this proposal. This is exactly the kind of function you'd expect to > see in the module. > > > > +1 from me. > > > > -bt > > > > On Sun, Feb 2, 2020 at 2:42 PM Jesse Claven <jesse...@gmail.com > <javascript:>> wrote: > > That all makes sense! I would say that in Elm it's used somewhat > frequently. I don't have access to the previous code that I worked on > (changed jobs), so unfortunately I'm unable to grep for `always` to find > some good examples. > > > > In the codebase at my new job, there's a couple of places where `fn _ -> > something` is (my original example in this thread). It's basically for > anywhere you'd want to ignore some value, and always return something else. > I tried searching through GitHub for that code sample but the search > functionality was a little subpar. > > > > I understand about keeping the stdlib small, but for a relatively small > function, and one that's considered "table stakes" in most FP languages, > perhaps it would be a good fit? > > > On Thursday, 30 January 2020 11:20:58 UTC, Wiebe-Marten Wijnja wrote: > > The reason `Function.identity/1` was added after it was requested many > times previously, was that at some point everyone agreed that it would > improve Elixir's documentation, because it is easier to search for than > `&(&1)`. > > The `const` pattern is much less wide-spread. In e.g. Haskell it sees some > use in places where it is the single (or at least by far the simplest) way > to make things typecheck in a pure functional environment. > > In Elixir, I suspect that it would be used much less commonly. The fact > that our functions contain statements that are executed from top to bottom > and rebindable variable names means that we are even less likely to use it > anywhere. > > As such, I don't think `const` is a function that is important enough to > include in the standard library. > > > > Of course, I'm very much open to evidence of the contrary ^_^. Do you have > any example code of where you'd see `const` being useful? > > > > ~Marten/Qqwy > > On 30-01-2020 10:24, Jesse Claven wrote: > > Hey Amos, > > > > Ah that's right. Every now and then I'm reminded that Elixir doesn't > support currying when I try to do something haha Your suggestion makes > sense. > > > > Would there be any downsides to having it, even if it isn't the most > incredible function? Again to call on the `Function.identity/1`, and it's > discussion, it's useful when it's useful for concise and semantic code. > > > On Wednesday, 29 January 2020 22:43:51 UTC, Amos King - Binary Noggin > wrote: > > Jesse, > > > > I see where you are going, but Elixir doesn't have currying. For instance, > the last example you gave will return an error. > > > > You could make it work by changing it a bit. > > > > `@spec const(a) :: fn(any() -> a) when a: var` > > > > So, const would return an fn instead of taking two arguments. I don't know > how useful that would be in the long run. I know it is common in function > programming, but I don't see it being extremely helpful in Elixir. > > > > I'm ready to be convinced. > > > > Cheers, > > > > Amos King > > CEO > > Binary Noggin > > http://binarynoggin.com #business > > https://elixiroutlaws.com #elixir podcast > > http://thisagilelife.com #podcast > > ======================================================= > > I welcome VSRE emails. Learn more at http://vsre.info/ > > ======================================================= > > > > > > On Wed, Jan 29, 2020 at 4:01 PM Jesse Claven <jesse...@gmail.com> wrote: > > Hey everyone! > > I propose adding `Function.const/2` as a function which for argument `x`, > would always return `x`. > > ``` > @spec const(any(), any()) :: any() > def const(_original_value, new_value), do: new_value > ``` > > This is somewhat similar to the new `Function.identity/1` in that it may > seem like we've easily lived without it, but it does afford some more > concise/semantic ways to represent a common pattern. > > ``` > fn _ -> something_else() end > ``` > > This would become: > > ``` > const(something_else()) > ``` > > ``` > Enum.map([0,1,2,3], &Function.const/2) > ``` > > I'm new to Elixir, so I'm not sure of the weight that the name `const` > would carry, so there could be a more acceptable name. > > If accepted, I'd be happy to create a PR! > > Equivalents in other languages: > > - Haskell: > https://hackage.haskell.org/package/base-4.12.0.0/docs/Data-Function.html#v:const > - PureScript: > https://pursuit.purescript.org/packages/purescript-const/4.1.0/docs/Data.Const > - Elm: https://package.elm-lang.org/packages/elm/core/latest/Basics#always > - Scala: > https://www.scala-lang.org/api/current/scala/Function$.html#const[T,U](x:T)(y:U):T > > - Idris: > https://www.idris-lang.org/docs/current/prelude_doc/docs/Prelude.Basics.html#Prelude.Basics.const > > > > Thanks for your time. > > > > -- > You received this message because you are subscribed to the Google Groups > "elixir-lang-core" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to elixir-l...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/elixir-lang-core/576949a3-93e2-4117-b1f1-ab4621f10e88%40googlegroups.com > > <https://groups.google.com/d/msgid/elixir-lang-core/576949a3-93e2-4117-b1f1-ab4621f10e88%40googlegroups.com?utm_medium=email&utm_source=footer> > . > > -- > You received this message because you are subscribed to the Google Groups > "elixir-lang-core" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to elixir-l...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/elixir-lang-core/f8b6009d-e21a-434a-96e0-e2c6ae3c1a0e%40googlegroups.com > > <https://groups.google.com/d/msgid/elixir-lang-core/f8b6009d-e21a-434a-96e0-e2c6ae3c1a0e%40googlegroups.com?utm_medium=email&utm_source=footer> > . > > -- > You received this message because you are subscribed to the Google Groups > "elixir-lang-core" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to elixir-l...@googlegroups.com <javascript:>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/elixir-lang-core/294d4f77-6b91-4316-a152-0c209fad12db%40googlegroups.com > > <https://groups.google.com/d/msgid/elixir-lang-core/294d4f77-6b91-4316-a152-0c209fad12db%40googlegroups.com?utm_medium=email&utm_source=footer> > . > > > > > -- > > > > Regards, > > Bruce Tate > > CEO > > > > [image: Image removed by sender.] > > > > Groxio, LLC. > > 512.799.9366 > > br...@grox.io <javascript:> > > grox.io > > -- > You received this message because you are subscribed to the Google Groups > "elixir-lang-core" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to elixir-l...@googlegroups.com <javascript:>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/elixir-lang-core/CAFXvW-6%3DfheYjjyQ7xiWYkP0B0t%2BfoGae9OGtkDDSvSpD4s_Sg%40mail.gmail.com > > <https://groups.google.com/d/msgid/elixir-lang-core/CAFXvW-6%3DfheYjjyQ7xiWYkP0B0t%2BfoGae9OGtkDDSvSpD4s_Sg%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > > -- You received this message because you are subscribed to the Google Groups "elixir-lang-core" group. To unsubscribe from this group and stop receiving emails from it, send an email to elixir-lang-core+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/elixir-lang-core/765cf782-6dc3-4b6c-a72f-974112182f53%40googlegroups.com.
