We continued our discussion of higher-order functions:
Recall that a higher order function is a function that acts on other functions, either taking them as inputs, or providing them as return values, or both.
We discussed combinators. Combinators are higher-order functions that lift operations from a type to functions with values in that type. Combinators may be defined using the pointwise principle. Currying makes this easy in SML.
We discussed staging. Staging takes advantage of the nested lambda form of a curried function to move parts of a computation close to the place where the arguments required for the computation appear. This can save computation time in a function that can do some of its work as soon as it sees its first argument, for instance. When called several times with the same first argument and different second arguments, the function can reuse the results of these early partial computations, since they are available in the environment part of the closure associated with the function that expects the second argument.
We talked about generalizing some higher-order list functions to trees.