swiftkeyword

What is the 'const' keyword in Swift?

I was reading this article and I came across the following two functions:

// Sequence actions, discarding the value of the second argument
func <* <A, B>(p: Parser<A>, q: Parser<B>) -> Parser<A> {
    return const <^> p <*> q
}

// Sequence actions, discarding the value of the first argument
func *> <A, B>(p: Parser<A>, q: Parser<B>) -> Parser<B> {
    return const(id) <^> p <*> q
}

What are the const and const(id)? I'm guessing they are some kind of values, but what values? Are they implicit left- or right-hand side operands? (This is just a shot in the dark). I could not find any info about it.

Solution

  • Swift has no const keyword.

    The talk uses the TryParsec library, which defines this const function:

    /// Constant function.
internal func const<A, B>(_ a: A) -> (B) -> A
{
    return { _ in a }
}

    In an expression like const(a)(b), the types of a and b can be anything. The expression evaluates a, then evaluates b, then discards the value of b and returns the value of a.

    It is modeled on the Haskell function const, which is itself an implementation of the K combinator.