How can i combine guards with pattern matching in Haskell? (And can I at all?)-CodePudding

Now I know that after I declare function signature with Haskell, I can do pattern matching with function overloading like so:

frog :: Int -> String
frog 1 = "Ribbit"
frog 7 = "Croak"
frog 12 = "Pop!"
frog x = replicate x "Z"

I know I also can use guards in a similar fashion:

frog :: Int -> String
frog x = 
    | x == 1 = "Ribbit"
    | x == 7 = "Croak"
    | x == 12 = "Pop!"
    | otherwise = replicate x "Z"

However I would rather prefer to combine the two ways using both a Boolean guard and a pattern to determine which arm would be executed. Something similar to this rust snippet:

fn frog(x: u32) -> String {
    match x {
        k if k >= 1000 => todo!()
        k if k >= 100 => todo!()
        k if k >= 10 => todo!()
        9 | 8 | 7 => todo!()
        6 | 5 | 4 => todo!()
        3 => todo!()
        2 => todo!()
        1 => todo!()
        0 => todo!()
    }
}

I would like to know if that's possible to do in Haskell, and if so how to do it. Thank you in advance

CodePudding user response：

You can't match multiple cases with a single pattern (the 9 | 8 | 7 thing), but you can straightforwardly combine patterns and guards:

foo 1 = "one"
foo 2 = "two"
foo k
    | k > 1000 = "greater than 1000"
    | k > 100 = "between 100 and 1000"
    | otherwise = "some other number"

The guards are part of the pattern - if none of the guards attached to a case succeed, then control falls through to the next case in the way you'd expect. (To put it another way, a single pattern's guards don't have to be exhaustive.)

bar k | k > 1000 = "greater than 1000"
bar _ = "smaller than 1000"