Question

I have a class named “Person”

Person :: String -> String -> Int -> Gender -> Person

Goal:
To make a more general version of Person, the femalePerson by assigning Gender to Female.

What works:
Binding first N elements works:

let personsWithNameAlice = Person "Alice"
let personsWithNameAliceMcGee = Person "Alice" "McGee"

What doesn’t work:

let femalePerson = Person {gender = Female}

Question:

• How do i do THIS?
• How do i a bind a value to n-th element of any function where n != 0?

It’s just that i know i can write f(x,y,z) = g(x,y,z,5) in math and std::bind2nd(f, 8) in C++ so shouldn’t it be possible in Haskell?

Answer 1

You can do it with a lambda expression, given

Person :: String -> String -> Int -> Gender -> Person

you’d define

femalePerson = \fn ln a -> Person fn ln a Female

or, with arguments in the binding:

femalePerson fn ln a = Person fn ln a Female

If you want to bind specifically the second parameter, flip is the function you want

third = flip (/) 3

You can use flip to bind arguments in arbitrary positions, but that quickly becomes a hassle and unreadable:

femalePerson = curry . curry $ flip (uncurry (uncurry Person)) Female