Elixir: error when piping to a function the result of inline comprehension

问题

I've noticed something surprising with Elixir's for comprehensions when used to pipe the results into a function.

For example, these forms work:

foo = fn(list) ->
  for n <- list do
    n + 1
  end
  |> Enum.reverse
end

foo.([1,2,3])
# [4, 3, 2]

foo = fn(list) ->
  for(n <- list, do: (n + 1))
  |> Enum.reverse
end

foo.([1,2,3])
# [4, 3, 2]

But this doesn't, as it considers the |> Mod.func on the second line part of the do block of the macro:

foo = fn(list) ->
  for n <- list, do: n + 1
  |> Enum.reverse
end

foo.([1,2,3])
** (Protocol.UndefinedError) protocol Enumerable not implemented for 2
    (elixir) lib/enum.ex:1: Enumerable.impl_for!/1
    (elixir) lib/enum.ex:116: Enumerable.reduce/3
    (elixir) lib/enum.ex:1636: Enum.reduce/3
    (stdlib) erl_eval.erl:670: :erl_eval.do_apply/6
    (stdlib) erl_eval.erl:228: :erl_eval.expr/5
    (elixir) lib/enum.ex:1623: Enum."-reduce/3-lists^foldl/2-0-"/3

I would guess that is has to do with how macros are expanded and their value returned, but interestingly these work:

bar = fn(list) ->
  if true, do: list
  |> Enum.reverse
end
bar.([1,2,3])
# [3, 2, 1]


bar = fn(list) ->
  if true, do: Enum.map(list, &(&1 + 1))
  |> Enum.reverse
end
bar.([1,2,3])
# [4, 3, 2]

回答1:

I think it's because of:

warning: you are piping into a function call without parentheses, which may be ambiguous. Please wrap the function you are piping into in parentheses.

This works:

iex()> foo = fn(list) ->
...()> (for n <- list, do: n + 1)
...()> |> Enum.reverse
...()> end
#Function<6.52032458/1 in :erl_eval.expr/5>
iex()> foo.([1,2,3])
[4, 3, 2]

without parentheses your function like:

foo = fn(list) ->
  for n <- list, do: n + 1 |> Enum.reverse
end

and n + 1 isn't a list so (Protocol.UndefinedError) protocol Enumerable not implemented for 2

来源：https://stackoverflow.com/questions/39402572/elixir-error-when-piping-to-a-function-the-result-of-inline-comprehension

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elixir