Is there a practical reason for <|> being left associative? In general, it's a control structure, where right-associativity is desirable (eg, what if if/then/else was left associative? It just doesn't make sense). For example, in all the canonical instance for parsers or Maybe, or Lists, left associativity leads to terrible evaluation characteristics and almost always requires more work than right-associativity. I always have to define a new local alias when writing parsers because it's a significant improvement.