Add two-fer exercise (#25)

config.json

@@ -115,6 +115,14 @@
         "difficulty": 3,
         "status": "beta"
       },
+      {
+        "slug": "two-fer",
+        "name": "Two Fer",
+        "uuid": "198c1176-536a-49db-828b-0dc1655d7da9",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 1,
+      },
       {
         "slug": "space-age",
         "name": "Space Age",

exercises/practice/two-fer/.approaches/config.json

@@ -0,0 +1,7 @@
+{
+  "introduction": {
+    "authors": ["BNAndras"],
+    "contributors": []
+  },
+  "approaches": []
+}

exercises/practice/two-fer/.approaches/introduction.md

@@ -0,0 +1,79 @@
+# Introduction
+
+There are generally two approaches for solving Two Fer.
+One approach would be using an [`if expression`][if-expression] to test a passed-in value.
+A variant of this approach would be an [`ask expression`][ask-expression] to concisely write the `if expression`.
+
+## General guidance
+
+Regardless of the approach used, Pyret doesn't allow one to [re-assign a variable binding][shadowing] that is already in scope
+Therefore, you can't rebind `name`.
+
+
+## Approach: `if` expressions
+
+```pyret
+fun two-fer(name):
+  fun get-name():
+    if name == "":
+      "you"
+    else:
+      name
+    end
+  end
+
+  "One for " + get-name() + ", one for me."
+end
+
+
+```
+
+## Approach: `ask` expressions
+
+```pyret
+fun two-fer(name):
+  fun get-name():
+    ask:
+      | name == "" then: "you"
+      | otherwise: name
+    end
+  end
+
+  "One for " + get-name() + ", one for me."
+end
+
+```
+
+## Other approaches
+
+Besides the aforementioned, idiomatic approaches, you could also approach the exercise as follows:
+
+### Other approach: Option datatypes
+
+Pyret provides a type of data called an [`Option`][option-datatype] that can represent a piece of data that is or is not available. The value `none` represents missing or invalid data and `some` wraps around data that is present. An empty incoming string `""` is still considered valid data so we'd need to replace it with `none`. Once we've done that, we can use [`or-else`][or-else] to either return the wrapped value or provide a default value when the Option is `none`.
+
+```pyret
+fun two-fer(name):
+  fun get-option():
+    if name == "":
+      none
+    else:
+      some(name)
+    end
+  end
+
+  option = get-option()
+  "One for " + option.or-else("you") + ", one for me."
+end
+
+```
+
+## Which approach to use?
+
+Either the `ask` or `if` expressions work well here and work similarly so the decision is largely stylistic.
+
+[if-expression]: https://pyret.org/docs/latest/Expressions.html#%28elem._%28bnf-prod._%28.Pyret._if-expr%29%29%29
+[ask-expression]: https://pyret.org/docs/latest/Expressions.html#%28part._s~3aask-expr%29
+[shadowing]: https://pyret.org/docs/latest/Bindings.html#%28part._s~3ashadowing%29
+[option-datatype]: https://pyret.org/docs/latest/option.html
+[or-else]: https://pyret.org/docs/latest/option.html#%28idx._%28gentag._342%29%29

exercises/practice/two-fer/.docs/instructions.md

@@ -0,0 +1,25 @@
+# Instructions
+
+Your task is to determine what you will say as you give away the extra cookie.
+
+If your friend likes cookies, and is named Do-yun, then you will say:
+
+```text
+One for Do-yun, one for me.
+```
+
+If your friend doesn't like cookies, you give the cookie to the next person in line at the bakery.
+Since you don't know their name, you will say _you_ instead.
+
+```text
+One for you, one for me.
+```
+
+Here are some examples:
+
+| Name   | Dialogue                    |
+| :----- | :-------------------------- |
+| Alice  | One for Alice, one for me.  |
+| Bohdan | One for Bohdan, one for me. |
+|        | One for you, one for me.    |
+| Zaphod | One for Zaphod, one for me. |

exercises/practice/two-fer/.docs/introduction.md

@@ -0,0 +1,8 @@
+# Introduction
+
+In some English accents, when you say "two for" quickly, it sounds like "two fer".
+Two-for-one is a way of saying that if you buy one, you also get one for free.
+So the phrase "two-fer" often implies a two-for-one offer.
+
+Imagine a bakery that has a holiday offer where you can buy two cookies for the price of one ("two-fer one!").
+You go for the offer and (very generously) decide to give the extra cookie to a friend.

exercises/practice/two-fer/.meta/config.json

@@ -0,0 +1,18 @@
+{
+  "authors": [
+    "BNAndras"
+  ],
+  "files": {
+    "solution": [
+      "two-fer.arr"
+    ],
+    "test": [
+      "two-fer-test.arr"
+    ],
+    "example": [
+      ".meta/example.arr"
+    ]
+  },
+  "blurb": "Create a sentence of the form \"One for X, one for me.\".",
+  "source_url": "https://github.com/exercism/problem-specifications/issues/757"
+}

exercises/practice/two-fer/.meta/example.arr

@@ -0,0 +1,7 @@
+provide: two-fer end
+
+fun two-fer(name):
+  ask:
+  | name == "" then: "One for you, one for me."
+  | otherwise: "One for " + name + ", one for me."
+end

exercises/practice/two-fer/.meta/tests.toml

@@ -0,0 +1,19 @@
+# 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.
+
+[1cf3e15a-a3d7-4a87-aeb3-ba1b43bc8dce]
+description = "no name given"
+
+[b4c6dbb8-b4fb-42c2-bafd-10785abe7709]
+description = "a name given"
+
+[3549048d-1a6e-4653-9a79-b0bda163e8d5]
+description = "another name given"

exercises/practice/two-fer/two-fer-test.arr

@@ -0,0 +1,13 @@
+include file("two-fer.arr")
+
+check "no name given":
+  two-fer("") is "One for you, one for me."
+end
+
+check "a name given":
+  two-fer("Alice") is "One for Alice, one for me."
+end
+
+check "another name given":
+  two-fer("Bob") is "One for Bob, one for me."
+end

exercises/practice/two-fer/two-fer.arr

@@ -0,0 +1,5 @@
+provide: two-fer end
+
+fun two-fer(name):
+  panic("Please implement the two-fer function")
+end