Complex sentence: This is a consequence of currying, which is made possible by Haskell's support for first-class functions; this function requires two inputs where one argument supplied and the function is "curried" to produce a function for the argument not supplied.
Split into simpler sentences: This is a consequence of currying, which is made possible by Haskell's support for first-class functions. That is, given a function that takes in two inputs, but had one argument supplied, the function is "curried" to produce a function of one argument--the one that is not supplied.