add some more

diff --git a/exercises/assignment5_answers.mdwn b/exercises/assignment5_answers.mdwn
index e7b8274..fd73778 100644 (file)
--- a/exercises/assignment5_answers.mdwn
+++ b/exercises/assignment5_answers.mdwn
@@ -291,7 +291,9 @@ Choose one of these languages and write the following functions.
             let (leader',left_score) = lh leader in
             let (leader'',right_score) = rh leader' in
             let my_score = left_score + right_score in
-            (match leader'' with | None -> Some(col,my_score), my_score | Some(_,leader_score) -> (if my_score > leader_score then Some(col,my_score) else leader''), my_score) in
+            (match leader'' with
+            | None -> Some(col,my_score), my_score
+            | Some(_,leader_score) -> (if my_score > leader_score then Some(col,my_score) else leader''), my_score) in
           tree_walker leaf_handler joiner t
 
     Then `tree_best` could be defined as in the direct answer.
@@ -327,11 +329,9 @@ That is, its leaves have no labels and its inner nodes are labeled with `int`s.
           let rec aux (trace : direction list) t =
             match t with
             | Nil -> None
-            | Inner(_,x,_) when x = sought -> Some(List.rev trace)
-            | Inner(l,_,r) ->
-              (match aux (Left :: trace) l with
-              | None -> aux (Right :: trace) r
-              | _ as result -> result) in
+            | Inner(l,x,r) when sought < x -> aux (Left::trace) l
+            | Inner(l,x,r) when sought > x -> aux (Right::trace) r
+            | _ -> Some(List.rev trace) in
         aux [] t
 
 
@@ -352,8 +352,18 @@ In question 2 above, you defined `maybe_map2`. [[Before|assignment1]] we encount
     <!-- or `f <$/fmap> ZipList xs <*/ap> ZipList ys`; or `pure f <*> ...`; or `liftA2 f (ZipList xs) (ZipList ys)` -->
     But we want you to write this function from scratch.)
 
+    HERE IS AN OCAML ANSWER:
+
+        let rec map2_zip f xs ys =
+          match xs, ys with
+          | [], _ -> []
+          | _, [] -> []
+          | x'::xs', y'::ys' -> f x' y' :: map2_zip f xs' ys'
+
 12. What is the relation between the function you just wrote, and the `maybe_map2` function you wrote for problem 2, above?
 
+    ANSWER: option/Maybe types are like lists constrained to be of length 0 or 1.
+
 13. Another strategy is to take the *cross product* of the two lists. If the function:
 
         (* OCaml *)
@@ -362,6 +372,13 @@ In question 2 above, you defined `maybe_map2`. [[Before|assignment1]] we encount
     is applied to the arguments `f`, `[x0, x1, x2]`, and `[y0, y1]`, then the result should be: `[f x0 y0, f x0 y1, f x1 y0, f x1 y1, f x2 y0, f x2 y1]`. Write this function.
     <!-- in Haskell, `liftA2 f xs ys` -->
 
+    HERE IS AN OCAML ANSWER:
+
+        let rec map2_cross f xs ys =
+          match xs with
+          | [] -> []
+          | x'::xs' -> List.append (List.map (f x') ys) (map2_cross f xs' ys)
+
 A similar choice between "zipping" and "crossing" could be made when `map2`-ing two trees. For example, the trees:
 
 <pre>
@@ -429,10 +446,11 @@ Again, we've left some gaps. (The use of `type` for the first line in Haskell an
 
         let rec occurs_free (ident : identifier) (term : lambda_term) : bool =
           match term with
-          | Var var_ident -> ident = var_indent (* `x` is free in Var "x" but not in Var "y" *)
-          | Abstract(bound_ident, term') -> ident <> bound_ident && occurs_free ident term' (* `x` is free in \y. x but not in \x. blah or \y. y *)
+          | Var var_ident -> ident = var_ident (* `x` is free in Var "x" but not in Var "y" *)
+          | Abstract(bound_ident, body) -> ident <> bound_ident && occurs_free ident body (* `x` is free in \y. x but not in \x. blah or \y. y *)
           | App (head, arg) -> occurs_free ident head || occurs_free ident arg
 
+  
 
 ## Encoding Booleans, Church numerals, and Right-Fold Lists in System F ##