Add strain exercise (#256)

config.json

@@ -74,6 +74,17 @@
           "floats"
         ]
       },
+      {
+        "slug": "strain",
+        "name": "Strain",
+        "uuid": "e929f6cf-04f8-41dd-b333-69be7b06d857",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 3,
+        "topics": [
+          "lists"
+        ]
+      },
       {
         "slug": "pangram",
         "name": "Pangram",
@@ -231,8 +242,10 @@
         "uuid": "c8b94fa9-d758-4dca-b7fc-7a130061e064",
         "practices": [],
         "prerequisites": [],
-        "difficulty": 1,
-        "topics": []
+        "difficulty": 4,
+        "topics": [
+          "lists"
+        ]
       },
       {
         "slug": "nth-prime",

exercises/practice/strain/.docs/instructions.md

@@ -0,0 +1,29 @@
+# Instructions
+
+Implement the `keep` and `discard` operation on collections.
+Given a collection and a predicate on the collection's elements, `keep` returns a new collection containing those elements where the predicate is true, while `discard` returns a new collection containing those elements where the predicate is false.
+
+For example, given the collection of numbers:
+
+- 1, 2, 3, 4, 5
+
+And the predicate:
+
+- is the number even?
+
+Then your keep operation should produce:
+
+- 2, 4
+
+While your discard operation should produce:
+
+- 1, 3, 5
+
+Note that the union of keep and discard is all the elements.
+
+The functions may be called `keep` and `discard`, or they may need different names in order to not clash with existing functions or concepts in your language.
+
+## Restrictions
+
+Keep your hands off that filter/reject/whatchamacallit functionality provided by your standard library!
+Solve this one yourself using other basic tools instead.

exercises/practice/strain/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "strain.sml"
+    ],
+    "test": [
+      "test.sml"
+    ],
+    "example": [
+      ".meta/example.sml"
+    ]
+  },
+  "blurb": "Implement the `keep` and `discard` operation on collections. Given a collection and a predicate on the collection's elements, `keep` returns a new collection containing those elements where the predicate is true, while `discard` returns a new collection containing those elements where the predicate is false.",
+  "source": "Conversation with James Edward Gray II",
+  "source_url": "http://graysoftinc.com/"
+}

exercises/practice/strain/.meta/example.sml

@@ -0,0 +1,17 @@
+fun keep f nil = nil
+  | keep f (first :: rest) =
+    let
+      val suffix = keep f rest
+    in
+      if f first then first :: suffix
+      else suffix
+    end
+
+fun discard f nil = nil
+  | discard f (first :: rest) =
+    let
+      val suffix = discard f rest
+    in
+      if f first then suffix
+      else first :: suffix
+    end

exercises/practice/strain/.meta/tests.toml

@@ -0,0 +1,52 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[26af8c32-ba6a-4eb3-aa0a-ebd8f136e003]
+description = "keep on empty list returns empty list"
+
+[f535cb4d-e99b-472a-bd52-9fa0ffccf454]
+description = "keeps everything"
+
+[950b8e8e-f628-42a8-85e2-9b30f09cde38]
+description = "keeps nothing"
+
+[92694259-6e76-470c-af87-156bdf75018a]
+description = "keeps first and last"
+
+[938f7867-bfc7-449e-a21b-7b00cbb56994]
+description = "keeps neither first nor last"
+
+[8908e351-4437-4d2b-a0f7-770811e48816]
+description = "keeps strings"
+
+[2728036b-102a-4f1e-a3ef-eac6160d876a]
+description = "keeps lists"
+
+[ef16beb9-8d84-451a-996a-14e80607fce6]
+description = "discard on empty list returns empty list"
+
+[2f42f9bc-8e06-4afe-a222-051b5d8cd12a]
+description = "discards everything"
+
+[ca990fdd-08c2-4f95-aa50-e0f5e1d6802b]
+description = "discards nothing"
+
+[71595dae-d283-48ca-a52b-45fa96819d2f]
+description = "discards first and last"
+
+[ae141f79-f86d-4567-b407-919eaca0f3dd]
+description = "discards neither first nor last"
+
+[daf25b36-a59f-4f29-bcfe-302eb4e43609]
+description = "discards strings"
+
+[a38d03f9-95ad-4459-80d1-48e937e4acaf]
+description = "discards lists"

exercises/practice/strain/strain.sml

@@ -0,0 +1,5 @@
+fun keep f l =
+  raise Fail "'keep' is not implemented"
+
+fun discard f l =
+  raise Fail "'discard' is not implemented"

exercises/practice/strain/test.sml

@@ -0,0 +1,110 @@
+(* version 1.0.0 *)
+
+use "testlib.sml";
+use "strain.sml";
+
+infixr |>
+fun x |> f = f x
+
+val testsuite =
+  describe "strain" [
+    test "keep on empty list returns empty list"
+      (fn _ => let
+        val l = []
+      in
+        keep (fn x => true) l |> Expect.equalTo []
+      end),
+
+    test "keeps everything"
+      (fn _ => let
+        val l = [1, 3, 5]
+      in
+        keep (fn x => true) l |> Expect.equalTo [1, 3, 5]
+      end),
+
+    test "keeps nothing"
+      (fn _ => let
+        val l = [1, 3, 5]
+      in
+        keep (fn x => false) l |> Expect.equalTo []
+      end),
+
+    test "keeps first and last"
+      (fn _ => let
+        val l = [1, 2, 3]
+      in
+        keep (fn x => x mod 2 = 1) l |> Expect.equalTo [1, 3]
+      end),
+
+    test "keeps neither first nor last"
+      (fn _ => let
+        val l = [1, 2, 3]
+      in
+        keep (fn x => x mod 2 = 0) l |> Expect.equalTo [2]
+      end),
+
+    test "keeps strings"
+      (fn _ => let
+        val l = ["apple", "zebra", "banana", "zombies", "cherimoya", "zealot"]
+      in
+        keep (String.isPrefix "z") l |> Expect.equalTo ["zebra", "zombies", "zealot"]
+      end),
+
+    test "keeps lists"
+      (fn _ => let
+        val l = [[1, 2, 3], [5, 5, 5], [5, 1, 2], [2, 1, 2], [1, 5, 2], [2, 2, 1], [1, 2, 5]]
+      in
+        keep (List.exists (fn x => x = 5)) l |> Expect.equalTo [[5, 5, 5], [5, 1, 2], [1, 5, 2], [1, 2, 5]]
+      end),
+
+    test "discard on empty list returns empty list"
+      (fn _ => let
+        val l = []
+      in
+        discard (fn x => true) l |> Expect.equalTo []
+      end),
+
+    test "discards everything"
+      (fn _ => let
+        val l = [1, 3, 5]
+      in
+        discard (fn x => true) l |> Expect.equalTo []
+      end),
+
+    test "discards nothing"
+      (fn _ => let
+        val l = [1, 3, 5]
+      in
+        discard (fn x => false) l |> Expect.equalTo [1, 3, 5]
+      end),
+
+    test "discards first and last"
+      (fn _ => let
+        val l = [1, 2, 3]
+      in
+        discard (fn x => x mod 2 = 1) l |> Expect.equalTo [2]
+      end),
+
+    test "discards neither first nor last"
+      (fn _ => let
+        val l = [1, 2, 3]
+      in
+        discard (fn x => x mod 2 = 0) l |> Expect.equalTo [1, 3]
+      end),
+
+    test "discards strings"
+      (fn _ => let
+        val l = ["apple", "zebra", "banana", "zombies", "cherimoya", "zealot"]
+      in
+        discard (String.isPrefix "z") l |> Expect.equalTo ["apple", "banana", "cherimoya"]
+      end),
+
+    test "discards lists"
+      (fn _ => let
+        val l = [[1, 2, 3], [5, 5, 5], [5, 1, 2], [2, 1, 2], [1, 5, 2], [2, 2, 1], [1, 2, 5]]
+      in
+        discard (List.exists (fn x => x = 5)) l |> Expect.equalTo [[1, 2, 3], [2, 1, 2], [2, 2, 1]]
+      end)
+  ]
+
+val _ = Test.run testsuite