Anonymous function shorthand ignores outer parens

[Ben Wilson]

Anonymous functions when present in pipes like the following

"this" |> &(&1).()

"this" |> &("#{&1} fails").()

All fail with errors like

# "this" |> &(&1).()

** (ArgumentError) cannot pipe "this" into &&1.(), can only pipe into local calls foo(), remote calls Foo.bar() or anonymous functions calls foo.()

    (elixir) lib/macro.ex:95: Macro.bad_pipe/2

    (stdlib) lists.erl:1261: :lists.foldl/3

    (elixir) expanding macro: Kernel.|>/2

** (ArgumentError) cannot pipe "this" into &<<Kernel.to_string(&1) :: binary, " fails">>.(), can only pipe into local calls foo(), remote calls Foo.bar() or anonymous functions calls foo.() (elixir) lib/macro.ex:95: Macro.bad_pipe/2 (stdlib) lists.erl:1261: :lists.foldl/3 (elixir) expanding macro: Kernel.|>/2 iex:3: (file)

This seems to indicate that the outer parens are being stripped for some reason. The following however works:

foo = &(&1); "this" |> foo.() #=> this

This looks like a bug to me, but I'm not entirely sure what kind (parser, expansion, etc).

- Ben

[José Valim]

When you write:

"this" |> &(&1).()

You are piping into the & operator. So Elixir is correctly complaining. We *could* fix this but I truly believe you should be rather using a named variable or a private function or at least wrapping the parenthesis externally:

"this" |> (& &1).()

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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[Ben Wilson]

AH right ok that makes sense.

[Hassan Zamani]

Why we can't pipe into & operator? e.i why Macro.pipe for {&, _, _} is a bad_pipe and not something like this:

def pipe(expr, {:&, _, _} = call_args) do
  call_args = {{:., [], [call_args]}, [], []}
  pipe(expr, call_args)
end

which will make the following work:

"This" |> &IO.puts/1
10 |> &(&1 + 10)

[José Valim]

Because pipe pipes into function calls, it doesn't pipe into functions. When you have &IO.puts/1, you have a function, you don't have a function call. It could behave as you proposed if we had currying but we don't so we need to stick with the macro semantics.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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[Hassan Zamani]

What is wrong with piping both to fuctions and fuction calls? What kind of bugs it may cause?

[Robert Virding]

What meaning would it have if you piped into a function? The whole of the pipe operator is to pass data between function calls.

Robert

[Hassan Zamani]

It would mean to call that function with given arg

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[Jason M Barnes]

A contrived example, but what if we have a couple of functions defined like so:

def func(), do: fun item -> … end

def func(item) do

  # do something with the item

end

Now we want to pipe something to the function:

:item |> func

If we allow piping to both functions and function calls, the meaning is ambiguous.  Do we mean to evaluate func/0 first to return the anonymous function, or do we pass :item directly to func/1?

I think limiting pipes to function calls keeps the code less ambiguous and easier to read.

Jason

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[Hassan Zamani]

As in the code above, by function I mean a capture, i.e. &func/1, and it specifies the ariry, so there will be no ambiguity.
Also that little peace of code do nothing but calling the capture, and as a result the code is nicer on the eye.

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[Jason M Barnes]

But then you’re typing more:

“This” |> &IO.puts/1

versus:

“This” |> IO.puts

I don’t see what this gets you.

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[José Valim]

The issue is that you are asking to special case *some functions*. Only functions that can be seen as functions during compile time would work. If you have a function at runtime, it won't work, and that will be ultimately confusing.

That's why pipe works on *calls*. Calls are compile time constructs by definition. You are asking for special cases to be added to a macro which is already very well defined.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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[Hassan Zamani]

Thanks. Now I am enlightened.

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