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+### when
+
+Tests a value, `x`, against a predicate function. If `true`, return `fn(x)`. Else, return `x`.
+`when(pred, whenTrue)` returns another function expecting `x` for better composability.
+
+```js
+const when = (pred, whenTrue) => (x) => pred(x) ? whenTrue(x) : x;
+```
+
+```js
+const doubleEvenNumbers = when(
+  (x) => x % 2 === 0,
+  (x) => x * 2
+);
+
+doubleEvenNumbers(2) // 4
+doubleEvenNumbers(1) // 1
+```