Lambda calculus is present in most "modern" programming languages, i.e. those which use lexical scope rather than global or dynamic scope; and those which call functions to return results (especially first-class functions) rather than jumping to subroutines for their effects. It's why Python functions are written with the keyword `lambda`, and it's why Amazon's function-as-a-service product is called Lambda.
For example, say you're refactoring some code and come across:
def foo(x):
return bar(x)
You decide to simplify this definition to:
foo = bar
Congratulations, you've just performed η-reduction!
https://en.wikipedia.org/wiki/Lambda_calculus#%CE%B7-reducti...