Add ListOps exercise (#605)

* Add ListOps exercise
* Explain `callables` in PHP
* ListOps: Implement all tests from problem spec

Author: homersimpsons

config.json

@@ -932,6 +932,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 2
+      },
+      {
+        "slug": "list-ops",
+        "name": "List Ops",
+        "uuid": "56f36086-96a8-4941-8c46-b563f02e5b09",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 6
       }
     ]
   },

exercises/practice/list-ops/.docs/instructions.append.md

@@ -0,0 +1,16 @@
+## Callable
+
+In PHP there is a concept of [callable](https://www.php.net/manual/en/language.types.callable.php).
+
+Those can take multiple forms, but we will focus on [anonymous functions](https://www.php.net/manual/en/functions.anonymous.php).
+
+It is possible to create an anonymous function in a variable and call it with parameters:
+
+```php
+$double = function ($number) {
+    return $number * 2;
+}
+
+$double(2); // returns 4
+$double(4); // returns 8
+```

exercises/practice/list-ops/.docs/instructions.md

@@ -0,0 +1,19 @@
+# Instructions
+
+Implement basic list operations.
+
+In functional languages list operations like `length`, `map`, and `reduce` are very common.
+Implement a series of basic list operations, without using existing functions.
+
+The precise number and names of the operations to be implemented will be track dependent to avoid conflicts with existing names, but the general operations you will implement include:
+
+- `append` (_given two lists, add all items in the second list to the end of the first list_);
+- `concatenate` (_given a series of lists, combine all items in all lists into one flattened list_);
+- `filter` (_given a predicate and a list, return the list of all items for which `predicate(item)` is True_);
+- `length` (_given a list, return the total number of items within it_);
+- `map` (_given a function and a list, return the list of the results of applying `function(item)` on all items_);
+- `foldl` (_given a function, a list, and initial accumulator, fold (reduce) each item into the accumulator from the left_);
+- `foldr` (_given a function, a list, and an initial accumulator, fold (reduce) each item into the accumulator from the right_);
+- `reverse` (_given a list, return a list with all the original items, but in reversed order_).
+
+Note, the ordering in which arguments are passed to the fold functions (`foldl`, `foldr`) is significant.

exercises/practice/list-ops/.meta/config.json

@@ -0,0 +1,15 @@
+{
+  "authors": ["homersimpsons"],
+  "files": {
+    "solution": [
+      "ListOps.php"
+    ],
+    "test": [
+      "ListOpsTest.php"
+    ],
+    "example": [
+      ".meta/example.php"
+    ]
+  },
+  "blurb": "Implement basic list operations."
+}

exercises/practice/list-ops/.meta/example.php

@@ -0,0 +1,99 @@
+<?php
+
+/*
+ * By adding type hints and enabling strict type checking, code can become
+ * easier to read, self-documenting and reduce the number of potential bugs.
+ * By default, type declarations are non-strict, which means they will attempt
+ * to change the original type to match the type specified by the
+ * type-declaration.
+ *
+ * In other words, if you pass a string to a function requiring a float,
+ * it will attempt to convert the string value to a float.
+ *
+ * To enable strict mode, a single declare directive must be placed at the top
+ * of the file.
+ * This means that the strictness of typing is configured on a per-file basis.
+ * This directive not only affects the type declarations of parameters, but also
+ * a function's return type.
+ *
+ * For more info review the Concept on strict type checking in the PHP track
+ * <link>.
+ *
+ * To disable strict typing, comment out the directive below.
+ */
+
+declare(strict_types=1);
+
+class ListOps
+{
+    public function append(array $list1, array $list2): array
+    {
+        foreach ($list2 as $el) {
+            $list1[] = $el;
+        }
+        return $list1;
+    }
+
+    public function concat(array $list1, array ...$listn): array
+    {
+        foreach ($listn as $list) {
+            $list1 = self::append($list1, $list);
+        }
+        return $list1;
+    }
+
+    public function filter(callable $predicate, array $list): array
+    {
+        $result = [];
+        foreach ($list as $el) {
+            if ($predicate($el)) {
+                $result[] = $el;
+            }
+        }
+        return $result;
+    }
+
+    public function length(array $list): int
+    {
+        $count = 0;
+        foreach ($list as $_el) {
+            $count++;
+        }
+        return $count;
+    }
+
+    public function map(callable $function, array $list): array
+    {
+        $result = [];
+        foreach ($list as $el) {
+            $result[] = $function($el);
+        }
+        return $result;
+    }
+
+    public function foldl(callable $function, array $list, $accumulator)
+    {
+        foreach ($list as $el) {
+            $accumulator = $function($accumulator, $el);
+        }
+        return $accumulator;
+    }
+
+    public function foldr(callable $function, array $list, $accumulator)
+    {
+        while (self::length($list) > 0) {
+            $el = array_pop($list);
+            $accumulator = $function($accumulator, $el);
+        }
+        return $accumulator;
+    }
+
+    public function reverse(array $list): array
+    {
+        $result = [];
+        while (self::length($list) > 0) {
+            $result[] = array_pop($list);
+        }
+        return $result;
+    }
+}

exercises/practice/list-ops/.meta/tests.toml

@@ -0,0 +1,112 @@
+# 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.
+
+[485b9452-bf94-40f7-a3db-c3cf4850066a]
+description = "append entries to a list and return the new list -> empty lists"
+
+[2c894696-b609-4569-b149-8672134d340a]
+description = "append entries to a list and return the new list -> list to empty list"
+
+[e842efed-3bf6-4295-b371-4d67a4fdf19c]
+description = "append entries to a list and return the new list -> empty list to list"
+
+[71dcf5eb-73ae-4a0e-b744-a52ee387922f]
+description = "append entries to a list and return the new list -> non-empty lists"
+
+[28444355-201b-4af2-a2f6-5550227bde21]
+description = "concatenate a list of lists -> empty list"
+
+[331451c1-9573-42a1-9869-2d06e3b389a9]
+description = "concatenate a list of lists -> list of lists"
+
+[d6ecd72c-197f-40c3-89a4-aa1f45827e09]
+description = "concatenate a list of lists -> list of nested lists"
+
+[0524fba8-3e0f-4531-ad2b-f7a43da86a16]
+description = "filter list returning only values that satisfy the filter function -> empty list"
+
+[88494bd5-f520-4edb-8631-88e415b62d24]
+description = "filter list returning only values that satisfy the filter function -> non-empty list"
+
+[1cf0b92d-8d96-41d5-9c21-7b3c37cb6aad]
+description = "returns the length of a list -> empty list"
+
+[d7b8d2d9-2d16-44c4-9a19-6e5f237cb71e]
+description = "returns the length of a list -> non-empty list"
+
+[c0bc8962-30e2-4bec-9ae4-668b8ecd75aa]
+description = "return a list of elements whose values equal the list value transformed by the mapping function -> empty list"
+
+[11e71a95-e78b-4909-b8e4-60cdcaec0e91]
+description = "return a list of elements whose values equal the list value transformed by the mapping function -> non-empty list"
+
+[613b20b7-1873-4070-a3a6-70ae5f50d7cc]
+description = "folds (reduces) the given list from the left with a function -> empty list"
+include = false
+comment = "Re-implemented in 36549237-f765-4a4c-bfd9-5d3a8f7b07d2"
+
+[e56df3eb-9405-416a-b13a-aabb4c3b5194]
+description = "folds (reduces) the given list from the left with a function -> direction independent function applied to non-empty list"
+include = false
+comment = "Re-implemented in 7a626a3c-03ec-42bc-9840-53f280e13067"
+
+[d2cf5644-aee1-4dfc-9b88-06896676fe27]
+description = "folds (reduces) the given list from the left with a function -> direction dependent function applied to non-empty list"
+include = false
+comment = "Re-implemented in d7fcad99-e88e-40e1-a539-4c519681f390"
+
+[36549237-f765-4a4c-bfd9-5d3a8f7b07d2]
+description = "folds (reduces) the given list from the left with a function -> empty list"
+reimplements = "613b20b7-1873-4070-a3a6-70ae5f50d7cc"
+
+[7a626a3c-03ec-42bc-9840-53f280e13067]
+description = "folds (reduces) the given list from the left with a function -> direction independent function applied to non-empty list"
+reimplements = "e56df3eb-9405-416a-b13a-aabb4c3b5194"
+
+[d7fcad99-e88e-40e1-a539-4c519681f390]
+description = "folds (reduces) the given list from the left with a function -> direction dependent function applied to non-empty list"
+reimplements = "d2cf5644-aee1-4dfc-9b88-06896676fe27"
+
+[aeb576b9-118e-4a57-a451-db49fac20fdc]
+description = "folds (reduces) the given list from the right with a function -> empty list"
+include = false
+comment = "Re-implemented in 17214edb-20ba-42fc-bda8-000a5ab525b0"
+
+[c4b64e58-313e-4c47-9c68-7764964efb8e]
+description = "folds (reduces) the given list from the right with a function -> direction independent function applied to non-empty list"
+include = false
+comment = "Re-implemented in e1c64db7-9253-4a3d-a7c4-5273b9e2a1bd"
+
+[be396a53-c074-4db3-8dd6-f7ed003cce7c]
+description = "folds (reduces) the given list from the right with a function -> direction dependent function applied to non-empty list"
+include = false
+comment = "Re-implemented in 8066003b-f2ff-437e-9103-66e6df474844"
+
+[17214edb-20ba-42fc-bda8-000a5ab525b0]
+description = "folds (reduces) the given list from the right with a function -> empty list"
+reimplements = "aeb576b9-118e-4a57-a451-db49fac20fdc"
+
+[e1c64db7-9253-4a3d-a7c4-5273b9e2a1bd]
+description = "folds (reduces) the given list from the right with a function -> direction independent function applied to non-empty list"
+reimplements = "c4b64e58-313e-4c47-9c68-7764964efb8e"
+
+[8066003b-f2ff-437e-9103-66e6df474844]
+description = "folds (reduces) the given list from the right with a function -> direction dependent function applied to non-empty list"
+reimplements = "be396a53-c074-4db3-8dd6-f7ed003cce7c"
+
+[94231515-050e-4841-943d-d4488ab4ee30]
+description = "reverse the elements of the list -> empty list"
+
+[fcc03d1e-42e0-4712-b689-d54ad761f360]
+description = "reverse the elements of the list -> non-empty list"
+
+[40872990-b5b8-4cb8-9085-d91fc0d05d26]
+description = "reverse the elements of the list -> list of lists is not flattened"

exercises/practice/list-ops/ListOps.php

@@ -0,0 +1,80 @@
+<?php
+
+/*
+ * By adding type hints and enabling strict type checking, code can become
+ * easier to read, self-documenting and reduce the number of potential bugs.
+ * By default, type declarations are non-strict, which means they will attempt
+ * to change the original type to match the type specified by the
+ * type-declaration.
+ *
+ * In other words, if you pass a string to a function requiring a float,
+ * it will attempt to convert the string value to a float.
+ *
+ * To enable strict mode, a single declare directive must be placed at the top
+ * of the file.
+ * This means that the strictness of typing is configured on a per-file basis.
+ * This directive not only affects the type declarations of parameters, but also
+ * a function's return type.
+ *
+ * For more info review the Concept on strict type checking in the PHP track
+ * <link>.
+ *
+ * To disable strict typing, comment out the directive below.
+ */
+
+declare(strict_types=1);
+
+class ListOps
+{
+    public function append(array $list1, array $list2): array
+    {
+        throw new \BadMethodCallException("Implement the append function");
+    }
+
+    public function concat(array $list1, array ...$listn): array
+    {
+        throw new \BadMethodCallException("Implement the concat function");
+    }
+
+    /**
+     * @param callable(mixed $item): bool $predicate
+     */
+    public function filter(callable $predicate, array $list): array
+    {
+        throw new \BadMethodCallException("Implement the filter function");
+    }
+
+    public function length(array $list): int
+    {
+        throw new \BadMethodCallException("Implement the length function");
+    }
+
+    /**
+     * @param callable(mixed $item): mixed $function
+     */
+    public function map(callable $function, array $list): array
+    {
+        throw new \BadMethodCallException("Implement the map function");
+    }
+
+    /**
+     * @param callable(mixed $accumulator, mixed $item): mixed $function
+     */
+    public function foldl(callable $function, array $list, $accumulator)
+    {
+        throw new \BadMethodCallException("Implement the foldl function");
+    }
+
+    /**
+     * @param callable(mixed $accumulator, mixed $item): mixed $function
+     */
+    public function foldr(callable $function, array $list, $accumulator)
+    {
+        throw new \BadMethodCallException("Implement the foldr function");
+    }
+
+    public function reverse(array $list): array
+    {
+        throw new \BadMethodCallException("Implement the reverse function");
+    }
+}