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+---
+title: when
+tags: function,intermediate
+---
+
+Tests a value, `x`, against a `predicate` function, conditionally applying a function. 
+
+Check if the value of `predicate(x)` is `True` and if so return `when_true(x)`, otherwise return `x`.
+
+```py
+def when(predicate, when_true):
+  return lambda x: when_true(x) if predicate(x) else x
+```
+
+```py
+double_even_numbers = when(lambda x: x % 2 == 0, lambda x : x * 2)
+double_even_numbers(2) # 4
+double_even_numbers(1) # 1
+```