What does “pure” mean in “pure functional language”?

Question

Haskell has been called a \"pure functional language.\"

What does \"pure\" mean in this context? What consequences does this have for a programmer?

Answer 1

Amr Sabry wrote a paper about what a pure functional language is. Haskell is by this definition considered pure, if we ignore things like unsafePerformIO. Using this definition also makes ML and Erlang impure. There are subsets of most languages that qualify as pure, but personally I don't think it's very useful to talk about C being a pure language.

Higher-orderness is orthogonal to purity, you can design a pure first-order functional language.