Some languages use the colon syntax, such as the Pascal family. Others, like Rust and Gleam, use the arrow syntax. The arrow syntax meshes better with the idea

Some languages use the colon syntax, such as the Pascal family. Others, like Rust and Gleam, use the arrow syntax. The arrow syntax meshes better with the idea that there's a difference between the types of values, and the meaning of a function signature. "foo: t" declares a name "foo" as having a type "t", but functions are abstractions — what is the "type" of factorial()? It's not "int". The type of factorial() could be written: (int) -> int or just: int -> int If you look at the ML family as well as Haskell, that's exactly how they express it. They do that for other reasons, too; those languages follow lambda calculus, which only has single-argument functions, so all functions are simply mappings between a single value to another value.