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Closure

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A closure, the opposite of a combinator, is a function that makes use of free variables in its definition. It 'closes' around some portion of its environment. for example

f x = (\y -> x + y)
f
returns a closure, because the variable
x
, which is bound outside of the lambda abstraction is used inside its definition. An interesting side note: the context in which
x
was bound shouldn't even exist anymore, and wouldn't, had the lambda abstraction not closed around x.