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+<h2>What are <code>when</code> Expressions?</h2> +<p>This is kinda where the whole idea for Baba Yaga started. Pattern matching is an approach to flow control. We do this in Baba Yaga using the <code>when</code> expression. It provides pattern matching functionality, allowing you to match values against patterns and execute different code based on the match.</p> +<pre class="prettyprint source lang-plaintext"><code>/* Pattern matching with when expressions */ +result : when x is + 0 then "zero" + 1 then "one" + _ then "other"; +</code></pre> +<p>Baba Yaga's pattern matching syntax has a lot of insporations, but especially <code>cond</code> patterns, Gleam's pattern matching, and Roc's, too.</p> +<h2>Basic Examples</h2> +<pre class="prettyprint source lang-plaintext"><code>/* Simple pattern matching */ +x : 5; +result : when x is + 0 then "zero" + 1 then "one" + 2 then "two" + _ then "other"; +/* Result: "other" */ + +/* Pattern matching with numbers */ +grade : 85; +letter_grade : when grade is + 90 then "A" + 80 then "B" + 70 then "C" + 60 then "D" + _ then "F"; +/* Result: "B" */ +</code></pre> +<h2>Pattern Types</h2> +<h3>Literal Patterns</h3> +<pre class="prettyprint source lang-plaintext"><code>/* Match exact values */ +result : when value is + true then "yes" + false then "no" + _ then "maybe"; +</code></pre> +<h3>Wildcard Pattern</h3> +<pre class="prettyprint source lang-plaintext"><code>/* _ matches anything */ +result : when x is + 0 then "zero" + _ then "not zero"; +</code></pre> +<h3>Function Reference Patterns</h3> +<pre class="prettyprint source lang-plaintext"><code>/* Match function references using @ operator */ +double : x -> x * 2; +square : x -> x * x; + +which : x -> when x is + @double then "doubling function" + @square then "squaring function" + _ then "other function"; + +test1 : which double; +test2 : which square; +</code></pre> +<p>As is called out elsewhere, too, the <code>@</code> operator is required when matching function references in patterns. This distinguishes between calling a function and matching against the function itself.</p> +<h3>Boolean Patterns</h3> +<pre class="prettyprint source lang-plaintext"><code>/* Match boolean values */ +result : when condition is + true then "condition is true" + false then "condition is false"; +</code></pre> +<h2>Complex Examples</h2> +<pre class="prettyprint source lang-plaintext"><code>/* Grade classification with ranges */ +score : 85; +grade : when score is + when score >= 90 then "A" + when score >= 80 then "B" + when score >= 70 then "C" + when score >= 60 then "D" + _ then "F"; +/* Result: "B" */ + +/* Multiple conditions */ +x : 5; +y : 10; +result : when x is + when x = y then "equal" + when x > y then "x is greater" + when x < y then "x is less" + _ then "impossible"; +/* Result: "x is less" */ +</code></pre> +<h2>Advanced Pattern Matching</h2> +<p>You can match multiple values with complex expressions:</p> +<pre class="prettyprint source lang-plaintext"><code>/* FizzBuzz implementation using multi-value patterns */ +fizzbuzz : n -> + when (n % 3) (n % 5) is + 0 0 then "FizzBuzz" + 0 _ then "Fizz" + _ 0 then "Buzz" + _ _ then n; + +/* Test the FizzBuzz function */ +result1 : fizzbuzz 15; /* "FizzBuzz" */ +result2 : fizzbuzz 3; /* "Fizz" */ +result3 : fizzbuzz 5; /* "Buzz" */ +result4 : fizzbuzz 7; /* 7 */ +</code></pre> +<p>You can access table properties directly in patterns:</p> +<pre class="prettyprint source lang-plaintext"><code>/* User role checking */ +user : {role: "admin", level: 5}; + +access_level : when user.role is + "admin" then "full access" + "user" then "limited access" + _ then "no access"; +/* Result: "full access" */ +</code></pre> +<p>You can use function calls in patterns. Be warned, though -- they require parentheses to help disambiguate them from other references, though.</p> +<pre class="prettyprint source lang-plaintext"><code>/* Even/odd classification */ +is_even : n -> n % 2 = 0; + +classify : n -> + when (is_even n) is + true then "even number" + false then "odd number"; + +/* Test the classification */ +result1 : classify 4; /* "even number" */ +result2 : classify 7; /* "odd number" */ +</code></pre> +<p>Function calls in patterns must be wrapped in parentheses!</p> +<p>This'll work:</p> +<pre class="prettyprint source lang-plaintext"><code>when (is_even n) is true then "even" +when (complex_func x y) is result then "matched" +</code></pre> +<p>This won't work:</p> +<pre class="prettyprint source lang-plaintext"><code>when is_even n is true then "even" /* Ambiguous parsing */ +</code></pre> +<p>You can nest <code>when</code> expressions for complex logic:</p> +<pre class="prettyprint source lang-plaintext"><code>/* Nested pattern matching */ +x : 5; +y : 10; +result : when x is + 0 then when y is + 0 then "both zero" + _ then "x is zero" + 1 then when y is + 1 then "both one" + _ then "x is one" + _ then when y is + 0 then "y is zero" + 1 then "y is one" + _ then "neither special"; +/* Result: "neither special" */ +</code></pre> +<h2>Using <code>when</code> with Functions</h2> +<pre class="prettyprint source lang-plaintext"><code>/* Function that uses pattern matching */ +classify_number : x -> when x is + 0 then "zero" + (x % 2 = 0) then "even" + (x % 2 = 1) then "odd" + _ then "unknown"; + +/* Use the function */ +result1 : classify_number 0; /* "zero" */ +result2 : classify_number 4; /* "even" */ +result3 : classify_number 7; /* "odd" */ +</code></pre> +<h2>Common Patterns</h2> +<pre class="prettyprint source lang-plaintext"><code>/* Value classification */ +classify_age : age -> when age is + (age < 13) then "child" + (age < 20) then "teenager" + (age < 65) then "adult" + _ then "senior"; + +/* Error handling */ +safe_divide : x y -> when y is + 0 then "error: division by zero" + _ then x / y; + +/* Status mapping */ +status_code : 404; +status_message : x -> + when x is + 200 then "OK" + 404 then "Not Found" + 500 then "Internal Server Error" + _ then "Unknown Error"; +</code></pre> +<h2>When to Use <code>when</code> pattern matching</h2> +<p><strong>Use <code>when</code> expressions when:</strong></p> +<ul> +<li>You need to match values against multiple patterns</li> +<li>You want to replace complex if/else chains</li> +<li>You're working with enumerated values</li> +<li>You need to handle different cases based on value types</li> +<li>You want to make conditional logic more readable</li> +<li><strong>You need to match multiple values simultaneously</strong> (multi-value patterns)</li> +<li><strong>You want to access table properties in patterns</strong> (table access)</li> +<li><strong>You need to use function results in patterns</strong> (function calls with parentheses)</li> +<li><strong>You're implementing complex validation logic</strong> (multi-field validation)</li> +<li><strong>You need to match function references</strong> (using <code>@</code> operator)</li> +</ul> +<p><strong>Don't use <code>when</code> expressions when:</strong></p> +<ul> +<li>You only have a simple true/false condition (use logical operators)</li> +<li>You're working with complex nested conditions (consider breaking into functions)</li> +</ul> +</article> + +</section> + +</div> + +<br class="clear"> + +<footer> + Generated by <a href="https://github.com/jsdoc3/jsdoc">JSDoc 4.0.4</a> on Tue Jul 29 2025 23:15:00 GMT-0400 (Eastern Daylight Time) using the Minami theme. +</footer> + +<script>prettyPrint();</script> +<script src="scripts/linenumber.js"></script> +</body> +</html> \ No newline at end of file |