X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=exercises%2Fassignment2_answers.mdwn;h=04c103c47aada473c00873f21eba7c69d90db46e;hp=177aa2d878aba67c242dd4e915e80b4462f18a5e;hb=7888d341776f23849c60cd82ca816ed13e5bc9c2;hpb=624df0618dbeb10a7b37e3dc9eb5c3f494650b03 diff --git a/exercises/assignment2_answers.mdwn b/exercises/assignment2_answers.mdwn index 177aa2d8..04c103c4 100644 --- a/exercises/assignment2_answers.mdwn +++ b/exercises/assignment2_answers.mdwn @@ -199,6 +199,7 @@ Folds and Lists reverse xs = fold_right (f, []) xs in reverse + *Here is an elegant, efficient answer following the [[hint|assignment2 hint]]* Suppose the list we want to reverse is `[10, 20, 30]`. Applying `fold_right` to this will begin by computing `f (30, z)` for some `f` and `z` that we specify. If we made the result of that be something like `30 & blah`, or any larger structure that contained something of that form, it's not clear how we could, using just the resources of `fold_right`, reach down into that structure and replace the `blah` with some other element, as we'd evidently need to, since after the next step we should get `30 & (20 & blah)`. What we'd like instead is something like this: @@ -237,7 +238,7 @@ Folds and Lists reverse xs = (fold_right (f, id) xs) [] in reverse - The ideas here are explored further in Chapter 8 of *The Little Schemer*. There they first introduce the idea of passing function as arguments to other functions, and having functions be the return values from functions. The `multirember&co` function discussed on pp. 137--140 (and the other `...&co` functions discussed in that chapter) are more complex examples of the kind of strategy used here to define `reverse`. We will be returning to these ideas and considering them more carefully later in the term. + The ideas here are explored further in Chapter 8 of *The Little Schemer*. There they first introduce the idea of passing function as arguments to other functions, and having functions be the return values from functions. Then the `multirember&co` function discussed on pp. 137--140 (and the other `...&co` functions discussed in that chapter) are more complex examples of the kind of strategy used here to define `reverse`. We will be returning to these ideas and considering them more carefully later in the term. Numbers