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fp/Terminology/Referential Transparency.md

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+A property of [[Purity|pure]] [[Functions|functions]] that they can always be replaced by the expression they return.
+
+For example `add` is referentially transparent, so
+```haskell
+a = add 1 3
+```
+
+could be replaced with
+```haskell
+a = 4
+```
+
+at design- or compile-time, because `add` always returns the same result and performs no observable side-effects.