[tromp]

Lambda calculus is as pure as can be, and also has terms that don't normalize. That is not considered a side effect.

A better example of impurity in Haskell for pragmatic's sake is the trace function, that can be used to print debugging information from pure functions.

[eru]

> Lambda calculus is as pure as can be, and also has terms that don't normalize. That is not considered a side effect.

Many typed lambda calculi do normalise. You can also have a look https://dhall-lang.org/ for some pragmatic that normalises.

> A better example of impurity in Haskell for pragmatic's sake is the trace function, that can be used to print debugging information from pure functions.

Well, but that's just unsafePerformIO (or unsafePerformIO-like) stuff under the hood; that was already mentioned.