up to 8 in week4

sml/week4/hw3.sml

@@ -1,6 +1,8 @@
 use "operators.sml";
 use "list.sml";
 
+exception NoAnswer
+
 fun first str = String.sub (str, 0)
 val only_capitals = filter (first >> Char.isUpper)
 
@@ -25,4 +27,25 @@ fun longest_string_helper f =
 
 val longest_capitalized = only_capitals >> longest_string2
 
-val rev_string = String.explode >> List.rev >> String.implode
+val rev_string = String.explode >> List.rev >> String.implode
+
+fun first_answer f lst =
+    case lst of
+       [] => raise NoAnswer
+     | head :: tail => 
+        case f head of
+        NONE => first_answer f tail
+        | SOME answer => answer
+
+fun all_answers (f: ('a -> 'b list option)) lst: 'b list option =
+    let
+        fun collect lst acc =
+            case lst of
+            [] => SOME acc
+            | head :: tail =>
+                case f head of
+                NONE => NONE
+                | SOME lst => collect tail (acc @ lst)
+    in
+        collect lst []
+    end

sml/week4/hw3_test.sml

@@ -44,5 +44,47 @@ val () =
     $ rev_string "string" = "gnirts"
     $ "rev_string: reverses string"
 
+val NONE =
+    first_answer (fn elm => NONE) []
+        handle NoAnswer =>
+            assert true "first_answer: rises on on empty lst" 
+            |> (fn _ => NONE)
+
+val NONE =
+    first_answer (fn elm => NONE) ["elm"]
+        handle NoAnswer =>
+            assert true "first_answer: rises on miss" 
+            |> (fn _ => NONE)
+
+val () =
+    assert
+    $ first_answer (fn elm => SOME elm) ["elm"] = "elm"
+    $ "first_answer: returns first answer"
+
+
+val () =
+    assert
+    $ first_answer 
+        (fn elm => if elm = "second" then SOME elm else NONE) 
+        ["elm", "second"] = "second"
+    $ "first_answer: returns some answer"
+
+val () =
+    assert
+    $ all_answers (fn elm => SOME elm) [] = SOME []
+    $ "all_answers: returns SOME [] on []"
+
+val () =
+    assert
+    $ all_answers (fn elm => SOME [elm]) ["a", "b"] = SOME ["a", "b"]
+    $ "all_answers: returns all answers"
+
+val () =
+    assert
+    $ all_answers 
+        (fn elm => if elm = "a" then SOME [elm] else NONE) 
+        ["a", "b"] = NONE
+    $ "all_answers: NONE on at least one NONE"
+
 
 val () = complete ()

sml/week4/list.sml

@@ -2,6 +2,8 @@ use "operators.sml";
 
 fun cons head tail = head :: tail
 
+fun append a b = a @ b
+
 fun foldl f acc lst =
     case lst of
     [] => acc
@@ -9,6 +11,7 @@ fun foldl f acc lst =
 
 fun reverse lst = foldl cons [] lst
 
+(* wrong implementation *)
 fun foldr f acc = foldl f acc >> reverse    
 
 fun map f = foldr (f >> cons) []