Add strain exercise (#297) (#306)

* Add generated files for new exercise strain

* Implement predicates in strain tests

* Add strain example

* Fix shadowing of the list built-in

* Avoid repetition in tests

* Use char=? instead of equal?

config.json

@@ -519,6 +519,14 @@
         "prerequisites": [],
         "difficulty": 1
       },
+      {
+        "slug": "strain",
+        "name": "Strain",
+        "uuid": "f3c6fea7-f69e-4cc5-b71f-0d34204623a7",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 2
+      },
       {
         "slug": "darts",
         "name": "Darts",

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": [
+    "Adrien-LUDWIG"
+  ],
+  "files": {
+    "solution": [
+      "strain.rkt"
+    ],
+    "test": [
+      "strain-test.rkt"
+    ],
+    "example": [
+      ".meta/example.rkt"
+    ]
+  },
+  "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.rkt

@@ -0,0 +1,11 @@
+#lang racket
+
+(provide keep discard)
+
+(define (keep lst predicate)
+  (for/list ([value lst]
+             #:when (predicate value))
+    value))
+
+(define (discard lst predicate)
+  (keep lst (negate predicate)))

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-test.rkt

@@ -0,0 +1,80 @@
+#lang racket/base
+
+(require "strain.rkt")
+
+(module+ test
+  (require rackunit rackunit/text-ui)
+
+  (define (always-true _) #t)
+
+  (define (always-false _) #f)
+
+  (define (contains-5? lst) (member 5 lst))
+
+  (define (starts-with-z? str) (char=? (string-ref str 0) #\z))
+
+  (define suite
+    (test-suite
+     "strain tests"
+
+     (test-equal? "keep on empty list returns empty list"
+                  (keep '() always-true)
+                  '())
+
+     (test-equal? "keeps everything"
+                  (keep '(1 3 5) always-true)
+                  '(1 3 5))
+
+     (test-equal? "keeps nothing"
+                  (keep '(1 3 5) always-false)
+                  '())
+
+     (test-equal? "keeps first and last"
+                  (keep '(1 2 3) odd?)
+                  '(1 3))
+
+     (test-equal? "keeps neither first nor last"
+                  (keep '(1 2 3) even?)
+                  '(2))
+
+     (test-equal? "keeps strings"
+                  (keep '("apple" "zebra" "banana" "zombies" "cherimoya" "zealot")
+                        starts-with-z?)
+                  '("zebra" "zombies" "zealot"))
+
+     (test-equal? "keeps lists"
+                  (keep (list '(1 2 3) '(5 5 5) '(5 1 2) '(2 1 2) '(1 5 2) '(2 2 1) '(1 2 5))
+                        contains-5?)
+                  (list '(5 5 5) '(5 1 2) '(1 5 2) '(1 2 5)))
+
+     (test-equal? "discard on empty list returns empty list"
+                  (discard '() always-true)
+                  '())
+
+     (test-equal? "discards everything"
+                  (discard '(1 3 5) always-true)
+                  '())
+
+     (test-equal? "discards nothing"
+                  (discard '(1 3 5) always-false)
+                  '(1 3 5))
+
+     (test-equal? "discards first and last"
+                  (discard '(1 2 3) odd?)
+                  '(2))
+
+     (test-equal? "discards neither first nor last"
+                  (discard '(1 2 3) even?)
+                  '(1 3))
+
+     (test-equal? "discards strings"
+                  (discard '("apple" "zebra" "banana" "zombies" "cherimoya" "zealot")
+                           starts-with-z?)
+                  '("apple" "banana" "cherimoya"))
+
+     (test-equal? "discards lists"
+                  (discard (list '(1 2 3) '(5 5 5) '(5 1 2) '(2 1 2) '(1 5 2) '(2 2 1) '(1 2 5))
+                           contains-5?)
+                  (list '(1 2 3) '(2 1 2) '(2 2 1)))))
+
+  (run-tests suite))

exercises/practice/strain/strain.rkt

@@ -0,0 +1,9 @@
+#lang racket
+
+(provide keep discard)
+
+(define (keep lst predicate)
+  (error "Not implemented yet"))
+
+(define (discard lst predicate)
+  (error "Not implemented yet"))