diff --git a/config.json b/config.json
index 1e65d29..e56e6f6 100644
--- a/config.json
+++ b/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",
diff --git a/exercises/practice/strain/.docs/instructions.md b/exercises/practice/strain/.docs/instructions.md
new file mode 100644
index 0000000..3469ae6
--- /dev/null
+++ b/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.
diff --git a/exercises/practice/strain/.meta/config.json b/exercises/practice/strain/.meta/config.json
new file mode 100644
index 0000000..ff594bf
--- /dev/null
+++ b/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/"
+}
diff --git a/exercises/practice/strain/.meta/example.sml b/exercises/practice/strain/.meta/example.sml
new file mode 100644
index 0000000..1f96965
--- /dev/null
+++ b/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
diff --git a/exercises/practice/strain/.meta/tests.toml b/exercises/practice/strain/.meta/tests.toml
new file mode 100644
index 0000000..3a617b4
--- /dev/null
+++ b/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"
diff --git a/exercises/practice/strain/strain.sml b/exercises/practice/strain/strain.sml
new file mode 100644
index 0000000..238a0e4
--- /dev/null
+++ b/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"
diff --git a/exercises/practice/strain/test.sml b/exercises/practice/strain/test.sml
new file mode 100644
index 0000000..9008be4
--- /dev/null
+++ b/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
diff --git a/exercises/practice/strain/testlib.sml b/exercises/practice/strain/testlib.sml
new file mode 100644
index 0000000..0c8370c
--- /dev/null
+++ b/exercises/practice/strain/testlib.sml
@@ -0,0 +1,160 @@
+structure Expect =
+struct
+  datatype expectation = Pass | Fail of string * string
+
+  local
+    fun failEq b a =
+      Fail ("Expected: " ^ b, "Got: " ^ a)
+    
+    fun failExn b a =
+      Fail ("Expected: " ^ b, "Raised: " ^ a)
+
+    fun exnName (e: exn): string = General.exnName e
+  in
+    fun truthy a =
+      if a
+      then Pass
+      else failEq "true" "false"
+
+    fun falsy a =
+      if a
+      then failEq "false" "true"
+      else Pass
+
+    fun equalTo b a = 
+      if a = b
+      then Pass
+      else failEq (PolyML.makestring b) (PolyML.makestring a)
+
+    fun nearTo delta b a =
+      if Real.abs (a - b) <= delta * Real.abs a orelse
+         Real.abs (a - b) <= delta * Real.abs b
+      then Pass
+      else failEq (Real.toString b ^ " +/- " ^ Real.toString delta) (Real.toString a)
+
+    fun anyError f =
+      (
+        f ();
+        failExn "an exception" "Nothing"
+      ) handle _ => Pass
+
+    fun error e f =
+      (
+        f ();
+        failExn (exnName e) "Nothing"
+      ) handle e' => if exnMessage e' = exnMessage e
+                     then Pass
+                     else failExn (exnMessage e) (exnMessage e')
+  end
+end
+
+structure TermColor =
+struct
+  datatype color = Red | Green | Yellow | Normal
+
+  fun f Red    = "\027[31m"
+    | f Green  = "\027[32m"
+    | f Yellow = "\027[33m"
+    | f Normal = "\027[0m"
+
+  fun colorize color s = (f color) ^ s ^ (f Normal)
+
+  val redit = colorize Red
+
+  val greenit = colorize Green
+
+  val yellowit = colorize Yellow
+end
+
+structure Test =
+struct
+  datatype testnode = TestGroup of string * testnode list
+                    | Test of string * (unit -> Expect.expectation)
+
+  local
+    datatype evaluation = Success of string
+                        | Failure of string * string * string
+                        | Error of string * string
+
+    fun indent n s = (implode (List.tabulate (n, fn _ => #" "))) ^ s
+
+    fun fmt indentlvl ev =
+      let
+        val check = TermColor.greenit "\226\156\148 " (* ✔ *)
+        val cross = TermColor.redit "\226\156\150 "   (* ✖ *)
+        val indentlvl = indentlvl * 2
+      in
+      case ev of
+        Success descr => indent indentlvl (check ^ descr)
+      | Failure (descr, exp, got) =>
+          String.concatWith "\n" [indent indentlvl (cross ^ descr),
+                                  indent (indentlvl + 2) exp,
+                                  indent (indentlvl + 2) got]
+      | Error (descr, reason) =>
+          String.concatWith "\n" [indent indentlvl (cross ^ descr),
+                                  indent (indentlvl + 2) (TermColor.redit reason)]
+      end
+
+    fun eval (TestGroup _) = raise Fail "Only a 'Test' can be evaluated"
+      | eval (Test (descr, thunk)) =
+          (
+            case thunk () of
+              Expect.Pass   => ((1, 0, 0), Success descr)
+            | Expect.Fail (s, s') => ((0, 1, 0), Failure (descr, s, s'))
+          )
+          handle e => ((0, 0, 1), Error (descr, "Unexpected error: " ^ exnMessage e))
+
+    fun flatten depth testnode =
+      let
+        fun sum (x, y, z) (a, b, c) = (x + a, y + b, z + c)
+
+        fun aux (t, (counter, acc)) =
+          let
+            val (counter', texts) = flatten (depth + 1) t
+          in
+            (sum counter' counter, texts :: acc)
+          end
+      in
+        case testnode of
+          TestGroup (descr, ts) =>
+            let
+              val (counter, texts) = foldr aux ((0, 0, 0), []) ts
+            in
+              (counter, (indent (depth * 2) descr) :: List.concat texts)
+            end
+        | Test _ =>
+            let
+              val (counter, evaluation) = eval testnode
+            in
+              (counter, [fmt depth evaluation])
+            end
+      end
+    
+    fun println s = print (s ^ "\n")
+  in
+    fun run suite =
+      let
+        val ((succeeded, failed, errored), texts) = flatten 0 suite
+
+        val summary = String.concatWith ", " [
+          TermColor.greenit ((Int.toString succeeded) ^ " passed"),
+          TermColor.redit ((Int.toString failed) ^ " failed"),
+          TermColor.redit ((Int.toString errored) ^ " errored"),
+          (Int.toString (succeeded + failed + errored)) ^ " total"
+        ]
+        
+        val status = if failed = 0 andalso errored = 0
+                     then OS.Process.success
+                     else OS.Process.failure
+
+      in
+        List.app println texts;
+        println "";
+        println ("Tests: " ^ summary);
+        OS.Process.exit status
+      end
+  end
+end
+
+fun describe description tests = Test.TestGroup (description, tests)
+fun test description thunk = Test.Test (description, thunk)