add find-min/max

tapl/tests/mlish/find.mlish

@@ -31,3 +31,43 @@
  : (Option Int)
  -> (None {Int}))
 
+(define (find-min/max [lst : (List X)] [<? : (→ Y Y Bool)] [extract-key : (→ X Y)]
+                      → (Option (× X X)))
+  (match lst with
+   [Nil -> None]
+   [Cons x1 rst ->
+         (let ([y1 (extract-key x1)])
+           (Some (find-min/max-accum rst <? extract-key x1 y1 x1 y1)))]))
+
+(define (find-min/max-accum [lst : (List X)] [<? : (→ Y Y Bool)] [extract-key : (→ X Y)]
+                            [x-min : X] [y-min : Y] [x-max : X] [y-max : Y]
+                            → (× X X))
+  (match lst with
+   [Nil -> (tup x-min x-max)]
+   [Cons x2 rst ->
+         (let ([y2 (extract-key x2)])
+           (cond [(<? y2 y-min)
+                  (find-min/max-accum rst <? extract-key x2 y2 x-max y-max)]
+                 [(<? y-max y2)
+                  (find-min/max-accum rst <? extract-key x-min y-min x2 y2)]
+                 [else
+                  (find-min/max-accum rst <? extract-key x-min y-min x-max y-max)]))]))
+
+(check-type
+ (find-min/max (Nil {Int})
+               (λ ([x : Int] [y : Int])
+                 (< x y))
+               (λ ([x : Int])
+                 x))
+ : (Option (× Int Int))
+ -> (None {Int}))
+
+(check-type
+ (find-min/max (Cons 1 (Cons 2 (Cons 3 Nil)))
+               (λ ([x : Int] [y : Int])
+                 (< x y))
+               (λ ([x : Int])
+                 x))
+ : (Option (× Int Int))
+ -> (Some (tup 1 3)))
+