Default function parameter ordering

Question

Reading through this, I came to the bit on default values for function arguments:

fill = (container, liquid = "coffee") ->
  "Filling the #{container} with #{liquid}..."

That's neat, but then I tried this:

fill = (container="mug", liquid = "coffee") ->
  "Filling the #{container} with #{liquid}..."

alert fill(liquid="juice")

and got the unexpected alert with "Filling the juice with coffee...". So then I tried this:

fill = (container="mug", liquid = "coffee") ->
  "Filling the #{container} with #{liquid}..."

alert fill(null, "juice")

and it worked. It's not pretty though. Is there a better way, or is this the idiomatic way to do this?

Answer 1

Currently, there's no way to call with named arguments. It would require knowing the arguments (names, positions, and/or default values) at the calling site, which is not always feasible in javascript/coffeescript.

Instead if you have many arguments and you want to name them and have default values, you could do something like this:

fill = (opts = {}) ->
    opts.container ?= "mug"
    opts.liquid ?= "coffee"
    "Filling the #{opts.container} with #{opts.liquid}..."

alert fill
    liquid:"juice"
    container:"cup"

alert fill
    liquid:"juice"

alert fill()

Answer 2

Amir and Jeremy already have this. As they point out, container="mug" in a function's argument list is really just shorthand for container ?= "mug" in the function body.

Let me just add that when calling functions,

fill(liquid="juice")

means the same thing as in JavaScript: First, assign the value "juice" to the liquid variable; then pass liquid along to fill. CoffeeScript doesn't do anything special here, and liquid has the same scope in that situation as it would outside of the function call.

By the way, I've suggested that the default argument syntax should be made more powerful by allowing arguments to be skipped (e.g. (first, middle ?= null, last) -> would assign values to first and last if only two arguments were passed), and that the ?= syntax should be used rather than =. You might want to express support for that proposal here: issue 1091.

Answer 3

fill = ({container, liquid} = {}) ->
     container ?= "mug"
     liquid ?= "coffee"

     "Filling the #{container} with #{liquid}..."

alert fill(liquid: "juice", container: "glass")
alert fill()

fill = (quantity="500 mL", {container, liquid} = {}) ->
     container ?= "mug"
     liquid ?= "coffee"

     "Filling the #{container} with #{quantity} of #{liquid}..."

alert fill("1L", liquid: "juice", container: "glass")
alert fill()
alert fill "1L"
alert fill "1L", liquid: "water"