[jashkenas]

I think it's safe to say. Arrow functions have existed in many different languages, but this particular flavor of using the fat arrow to declare a "lexical `this`" function, where the value of "this" within the function body preserves the value of "this" outside, is in CoffeeScript:

http://coffeescript.org/#fat_arrow

[eric_bullington]

Jeremy, would you mind sharing what the inspiration was for CoffeeScript's fat arrow? Was that token inspired by another language or was it something you just came up with on your own?

[jashkenas]

Sure -- the syntax for arrow functions has been floating around for ages, in Haskell, C#, and probably most relevant for me, the "stabby lambdas" in Ruby 1.9.

One of the first ideas with CoffeeScript was to simplify function syntax, mostly because JavaScript's anonymous functions are so useful and pervasive that you end up with a lot of this:

    _.each(toRemove, function(key){ delete data[key]; });

... where the "word" function there doesn't really tell you anything meaningful about what you're trying to accomplish.

Arrows were appealing for a shorthand function syntax because of their visual representation of what happens in a (pure) function. Input goes in one side, output comes out the other. The input determines, or points to, the output. Since we use parentheses symmetrically to group parameters to a function, both when you define a function and when you call one, we arrive at this:

    (input) -> output

... or with our initial example:

    _.each toRemove, (key) -> delete data[key]

That was the thinking. CoffeeScript actually originally used => for all functions, but the fat arrow distinction was introduced when we introduced bound (lexical "this") functions as an alternative to normal (dynamic "this") functions.

[eric_bullington]

Thanks for the detailed response. I thought it might have been inspired from Haskell or Ruby, but was surprised to find out that C# has a similar syntactic feature. So I was curious if there was an "official" etymology. Sounds like an amalgamation of all of the above.