Detect repeat argument names at compile time

[rudi-c]

The following code

sum x x = x + x
mySum = sum 1 2

Generates the error

Uncaught SyntaxError: Strict mode function may not have duplicate parameter names

Which is good, except that the error message appears only in the console. Instead, the webpage only shows

Cannot read property 'make' of undefined
    Open the developer console for more details.

Which makes debugging really confusing.

[thSoft]

This has bit me multiple times! Also happens when declaring a local variable with the same name as an argument:

update action model =
  let result =
        (model, effects)
      (updatedSubmodel, effects) =
        Subcomponent.update action model.submodel
      model = -- should be updatedModel
        { model | submodel <- updatedSubmodel }
  in result

[jvoigtlaender]

Are the previous two comments really placed correctly in this issue here? Don't they belong to #829 / #1018 instead?

Look at the opening comment of this issue. It is about a very specific situation that is not what you (@thSoft and @Apanatshka) are talking about here.

[thSoft]

Sorry, you're right, @jvoigtlaender. Should I delete my comment?

[mgold]

As for the original issue, I remember seeing something similar. Can someone please verify that the issue exists in the 0.16 alpha?

[mgold]

It still exists in 0.16-alpha3. In the repl:

> sum x x = x + x
/<PATH SNIPPED>/repl-temp-000.js:6039
   var sum = F2(function (x,x) {    return x + x;});
                            ^

SyntaxError: Duplicate parameter name not allowed in this context
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:404:25)
    at Object.Module._extensions..js (module.js:432:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:311:12)
    at Function.Module.runMain (module.js:457:10)
    at startup (node.js:136:18)
    at node.js:972:3

[yblein]

Also the following code

sum x (x, y) = x + y
mySum = sum 1 (2, 3)

does not produce an error. Instead, mySum is equal to 5.
I guess this kind of multiple binding should be prevented too?

[mgold]

Yes, there is no sensible way to interpret that definition.