Defining infix operators in elixir?

[Ian Asaff]

I was looking through the source for the Kernal module. In looking at the code for various operators (take ">" for example), I was confused by the syntax.

The source (https://github.com/elixir-lang/elixir/blob/34df8176385200d483ab483a973c498ffa62e580/lib/elixir/lib/kernel.ex#L936) seems to imply that it's easy to implement an infix operator using the following syntax (where "op" is the operator). 

def left op right do

  # some code that does stuff given two parameters

end

so ">" is implemented like so:

def left > right do

  :erlang.>(left,right)

end

What's special about the Kernal module that allows that type of function definition? Did I miss something in the docs that specifies?

[Alexei Sholik]

One special thing about Kernel is that it defines the original implementations for each operator. You can redefine any of the operators if you first unimport it inside your module.

Here's an example https://github.com/alco/pipespect/blob/master/lib/pipespect.ex#L9-L10

It can be a function or a macro depending on whether you want to delay the evaluation of the arguments or not.

Once you have a new implementation for an operator (say, in module called MyOp), you can call it as a function

MyOp.>(a, b)

or you can import it and use it as an operator

import Kernel, except: [>: 2]
import MyOp

a > b

To make it simpler, one usually defines a __using__ macro that does the unimport and import, then in the user code one just needs to write

use MyOp

a > b

See the link above for an example of this.

There are some operators that don't have implementations by default. Those can be implemented without unimport from Kernel – https://github.com/elixir-lang/elixir/blob/0b1c3b7d284da4b0b99b4f0cc172d6e37f6c43ea/lib/elixir/lib/macro.ex#L20

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[Ian M. Asaff]

Alexei,

Thanks so much for the detailed explanation. It's much appreciated!

Ian

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[Roger Qiu]

Does Elixir support custom operators yet?

[José Valim]

No, and it isn't scheduled as a feature for now. There are some operators available though (they are not used anywhere in the stdlib) and Alexei linked to them.

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Founder and Lead Developer