Tuesday, 15 September 2015

Explaining closure pattern name

Live and learn. Indeed.

I'm currently reading a book about Scala and found that "closure" "design pattern" which I've been using in JavaScript for ages is actually from functional programming world, and this closure is not a whole thing, but a variable closing so called "open term".

Imagine the function:
function myFunc(a) {
  return a + b
}

The "a" variable here is a "bound variable", and it makes sense in context of myFunc. The "b" variable is a "free variable" and it is senseless in this context. The "a + b" expression here is an "open term". On the other hand if we replace the expression in myFunc with something like "a + 2" it will be a "closed term".

The name "closure" arises from the act of "closing" the function literal with the open term(s) by "capturing" bindings of its free variables.

var b = 2 //this b variable is actually a closure!
function myFunc(a) {
  return a + b
}

That's it. This variable is the "closure". And the thing is the "function literal" with the "open term".

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