Composition, lambda, and section

by Xinya

I will discuss the difference between the following two code snippets.

> (define numwords
    (lambda (str)
      (length (string-split str " "))))

and

> (define numwords (o length (section string-split <> " ")))

Will the second one work without (o )? Why can the first one work without composition? It is worth noticing that composition is for two functions, or in CS sense, two procedures. In the first one, (string-split str " ") in lambda is a complete statement, which turns out to be a value as the parameter for (length number). However, for the second one, (section string-split <> " ") is still a procedure. You can test it by typing (section string-split <> " ") in the lower window and DrRacket should tell you this is a procedure. Therefore, in order to bridge bewteen (section string-split <> " ") and length which is a procedure itself, you need to use composition (aka (o proc1 proc2)) which takes two procedures as parameters.