[runeks]

You’re mixing up laziness and purity. Purity makes side effects impossible, so to speak, not laziness.

Haskell is the most functional language because a Haskell function is a mathematical function, which can only transform its arguments into a value. Everything is a constant in Haskell, and functions transform one or more constants into a single, new constant.

[tome]

No he/she isn't. The argument (as advanced by SPJ) is that it is so awkward for a programmer to use side-effects in a lazy language that it is (practically, not theoretically) impossible for a language designer to add them.