Add more leap approaches (#180)

exercises/practice/leap/.approaches/boolean-chain/content.md

@@ -10,7 +10,7 @@ fun leap(year):
 end
 ```
 
-This approach uses the Boolean operators `and` and `or` to chain together values from two Boolean expressions, creating a Boolean value.
+This approach uses a helper function `year-is-divisible-by` and the Boolean operators `and` and `or` to run each check (divisible by 4, not divisible by 100, and divisible by 400) in order, producing a single Boolean value at the end.
 The `and` operator returns `true` when both sides are true but `false` otherwise.
 The `or` operator returns `false` when both sides are false but `true` otherwise.
 

exercises/practice/leap/.approaches/boolean-chain/snippet.txt

@@ -1 +1,7 @@
-PLACEHOLDER
+fun leap(year):
+  fun year-is-divisible-by(divisor):
+    num-equal(num-modulo(year, divisor), 0)
+  end
+
+  year-is-divisible-by(4) and (not(year-is-divisible-by(100)) or year-is-divisible-by(400))
+end

exercises/practice/leap/.approaches/config.json

@@ -9,7 +9,16 @@
       "uuid": "2f53896f-b3e7-4c7d-be3a-a3342087826e",
       "slug": "boolean-chain",
       "title": "Boolean Chaining",
-      "blurb": "Use operators to check boolean values in a chain",
+      "blurb": "Use operators to check Boolean values in a chain",
+      "authors": [
+        "BNAndras"
+      ]
+    },
+    {
+      "uuid": "19b192ea-7dbe-4c22-b29c-93df1127f7e0",
+      "slug": "if-expressions",
+      "title": "If Expressions",
+      "blurb": "Use if expressions to conditionally check a leap year",
       "authors": [
         "BNAndras"
       ]

exercises/practice/leap/.approaches/if-expressions/content.md

@@ -0,0 +1,32 @@
+# If expressions
+
+```pyret
+fun leap(year):
+  if num-equal(num-modulo(year, 400), 0):
+    true
+  else if num-equal(num-modulo(year, 100), 0):
+    false
+  else if num-equal(num-modulo(year, 4), 0):
+    true
+  else:
+    false
+end
+```
+
+An [if expression][if-expression] contains one or more branches with conditions that are checked sequentially and associated blocks of code.
+Once a condition produces `true`, the associated block is evaluated and its contents returned.
+No subsequent conditions will be tested.
+One of the conditions in the if expression must produce `true` or an error is thrown.
+Therefore, `else` is used to provide a branch that is evaluated when the other branches' conditions aren't satisified.
+
+If a year is evenly divisible by 400, the first condition `num-equal(num-modulo(year, 400), 0)` produces `true`, and the associated block is evaluated, returning `true`.
+
+If a year is divisible by 100, the second condition `num-equal(num-modulo(year, 100), 0)` produces `true`, and the associated block is evaluated, returning `false`. If this year had also been divisible by 400, the first condition would have been satisfied and the code would not have reached this point.
+
+If a year is divisible by 4, the third condition `num-equal(num-modulo(year, 4), 0)` produces `true`, and the associated block is evaluated, returning `true`.
+Because of the previous conditions, this year isn't also divisible by 400 or 100.
+
+Finally, if no other branches' conditions were true, the block associated with `else` is evaluated, representing when a year isn't divisible by 4, 100, or 400.
+Here, `false` is returned.
+
+[if-expression]: https://pyret.org/docs/latest/Expressions.html#%28part._s~3aif-expr%29

exercises/practice/leap/.approaches/if-expressions/snippet.txt

@@ -0,0 +1,8 @@
+fun leap(year):
+  if num-equal(num-modulo(year, 400), 0):
+    true
+  else if num-equal(num-modulo(year, 100), 0):
+    false
+  else if num-equal(num-modulo(year, 4), 0):
+    true
+# clipped for brevity

exercises/practice/leap/.approaches/introduction.md

@@ -21,5 +21,23 @@ end
 
 For more information, check the [Boolean chain approach][approach-boolean-chain].
 
+## Approach: If expressions
+
+```pyret
+fun leap(year):
+  if num-equal(num-modulo(year, 400), 0):
+    true
+  else if num-equal(num-modulo(year, 100), 0):
+    false
+  else if num-equal(num-modulo(year, 4), 0):
+    true
+  else:
+    false
+end
+```
+
+For more information, check the [if expressions approach][approach-if-expressions].
+
 [num-modulo]: https://pyret.org/docs/latest/numbers.html#%28part._numbers_num-modulo%29
-[approach-boolean-chain]: https://exercism.org/tracks/pyret/exercises/leap/approaches/boolean-chain
+[approach-boolean-chain]: https://exercism.org/tracks/pyret/exercises/leap/approaches/boolean-chain
+[approach-if-expressions]: https://exercism.org/tracks/pyret/exercises/leap/approaches/if-expressions