dates_in_month done

sml/week1/date.sml

@@ -10,10 +10,28 @@ fun fold f lst acc =
         [] => acc
         | head :: tail => fold f tail (f head acc)
 
-fun is_older (
-    (y1, m1, d1): Date,
-    (y2, m2, d2): Date
-): bool =
+fun reverse lst =
+    fold (fn elm => fn acc => elm :: acc) lst []
+
+
+(* non tail optimized :( *)
+(* fun filter predicate lst =
+    case lst of
+       [] => lst
+     | head :: tail => 
+        if predicate head
+        then head :: filter predicate tail
+        else filter predicate tail *)
+
+fun filter predicate lst = 
+    let
+        val reversed = reverse lst
+        fun f elm acc = if predicate elm then elm :: acc else acc
+    in
+        fold f reversed []
+    end
+
+fun is_older ((y1, m1, d1): Date, (y2, m2, d2): Date): bool =
         let
             val same_dates = y1 = y2 andalso m1 = m2 andalso d1 = d2
             val older = y1 <= y2 andalso m1 <= m2 andalso d1 <= d2
@@ -21,10 +39,7 @@ fun is_older (
             if same_dates then false else older
         end
 
-fun number_in_month (
-    dates: Date list,
-    month_to_find: int
-): int =
+fun number_in_month (dates: Date list, month_to_find: int): int =
     let         
         fun count_month (_, month, _) occurences =  
             if month = month_to_find
@@ -34,13 +49,17 @@ fun number_in_month (
         fold count_month dates 0
     end
 
-fun number_in_months (
-    dates: Date list,
-    months_to_find: int list
-): int =
+fun number_in_months (dates: Date list, months_to_find: int list): int =
     let
-        fun count_months month acc = acc + number_in_month (dates, month)
+        fun count_months month acc = 
+            acc + number_in_month (dates, month)
     in
         fold count_months months_to_find 0
     end
 
+fun dates_in_month (dates: Date list, in_month: int): Date list =
+    let
+        fun filter_dates (_, month, _) = in_month = month
+    in
+        filter filter_dates dates
+    end

sml/week1/date_tests.sml

@@ -97,4 +97,26 @@ val () =
             "number_in_months: multiple dates with same month"
     end
 
+val () =
+    let
+        val dates = []
+        val month = 12
+        val expect = []
+    in
+        assert
+            (dates_in_month (dates, month) = expect)
+            "dates_in_month: returns empty list on empty dates"
+    end
+
+val () =
+    let
+        val dates = [(2000, 11, 31), (2000, 12, 30),  (2000, 12, 31)]
+        val month = 12
+        val expect = [(2000, 12, 30),  (2000, 12, 31)]
+    in
+        assert
+            (dates_in_month (dates, month) = expect)
+            "dates_in_month: filter non matching dates"
+    end
+
 val () = complete ()