Parenthesis and nested calls

[Torben Hoffmann]

Hi,

I tried to run this:
iex(42)> [ integer_to_binary (5)]
["5"]
iex(43)> [ integer_to_binary (5), integer_to_binary(2)]
** (SyntaxError) iex:43: unexpected comma. Parentheses are required to solve ambiguity in nested calls. Syntax error before: ','

That seems a bit weird to me?!?! Is is not allowed to have a space between the function name and the ( in a list?

Cheers,
Torben

[Peter Minten]

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It is a bit weird. Elixir's optional parentheses are what's causing
this weirdness.

Thing is, for the compiler you could have intended two things:

[ integer_to_binary((5), integer_to_binary(2))]
[ integer_to_binary(5), integer_to_binary(2)]

This is due to "(5)" being a valid expression in it's own right
(parentheses to override the default precedence rules).

Instead of choosing one or the other it complains forcing you to make
the choice explicitly.

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[Eric Meadows-Jönsson]

Elixir errors here because there is an ambiguity. It can be parsed as [ integer_to_binary( (5), integer_to_binary(2) ) ] or as [ integer_to_binary((5)), integer_to_binary(2) ].

Elixir has optional parenthesis for function calls, so when it sees a space after the function name it thinks you are calling the function with no parenthesis and the first argument will be parsed as an ordinary expression enclosed in parenthesis.

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Eric Meadows-Jönsson

[Alexei Sholik]

Try without a list:

iex(8)> integer_to_binary (5), integer_to_binary(2)

** (ArgumentError) argument error

    :erlang.integer_to_binary(5, "2")

Parentheses are implicit. "integer_to_binary (5)" is not separating paren from a function name, it's calling a function with the argument "(5)". So the invocation is parsed as integer_to_binary(5, integer_to_binary(2)). You can see that in the stacktrace.

You get a different error inside a list because that is a special case for that defined in the parser. I think the motivation is to avoid ambiguity – does this comma separate list elements or function arguments? See these tests https://github.com/elixir-lang/elixir/blob/master/lib/elixir/test/elixir/kernel/errors_test.exs#L80

When in doubt, use parentheses in calls, i.e. 

iex(8)> [integer_to_binary(5), integer_to_binary(2)]

["5", "2"]

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Best regards
Alexei Sholik

[José Valim]

Hello Torben!

When you write integer_to_binary (5) with a space in between, regardless if inside a list or not, Elixir treats that as invoking integer_to_binary/1 passing the (5) expression as argument. In order to make it clear:

iex> integer_to_binary(5, 2)

"101"

iex> integer_to_binary (5, 2)

** SyntaxError

The second example fails because (5, 2) is considered to be an expression (and an invalid one) and not arguments to integer_to_binary. Of course we could make it work but we often avoid such ambiguous cases to force everyone to share a similar code style. The only reason "integer_to_binary (5)" works is because "integer_to_binary EXPR" is a valid call anywhere.

TL;DR: Do not add a space in between the function name and its (arguments)

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Lead Developer

On Fri, Oct 11, 2013 at 11:35 AM, Torben Hoffmann <torben...@gmail.com> wrote:

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[Torben Hoffmann]

Thanks for all the replies!

It's just one of those things than one needs to get used to.

Cheers,
Torben