CODEWITHSUNDEEP
haskell
Cardano
Cardano

Reputation: 462

Passing list to another function

I have this code which generates prime numbers from 2 to n

primes :: Integer -> [Integer]
primes n = sieve [2..n]
where
 sieve (p:xs) = p : sieve [x|x <- xs, x `mod` p > 0]

How do I pass the sieved list (which has my primes) to another function?

Something like this doesn't work (parse error)

someText:: [Integer] -> String
someText (x:[]) = "List starts  with" ++ show x

primes :: Integer -> [Integer]
primes n = sieve [2..n]
where
 sieve (p:xs) = p : sieve [x|x <- xs, x `mod` p > 0]
 someText sieve

Upvotes: 1

Views: 549

Answers (1)

leftaroundabout
leftaroundabout

Reputation: 120711

You don't pass something to a function, like some list. That's imperative thinking. What you do in Haskell is, you evaluate some function with e.g. a list as its argument. Normally, you'd just leave primes as it is and use the result as the argument for someText. Like†,

main :: IO ()
main = print (someText $ primes 100)

or simply in the REPL

GHCi> someText $ primes 100
"List starts  with2"

Of course, you can also redefine primes to include someText. But obviously, the result will then be a string and not a list anymore!

primes' :: Integer -> String
primes' n = someText $ sieve [2..n]
 where sieve (p:xs) = p : sieve [x|x <- xs, x `mod` p > 0]

It's probably better not to do this, because such a string is pretty useless when you want to use the prime list in any other way. You can better just invoke the original primes function:

primes' :: Integer -> String
primes' = someText . primes

†Actually this doesn't work: your function someText isn't complete. The pattern x:[] only matches a list which has exactly one element (this is more handily written someText [x] = ...). You want to match on any list which starts with one element; do that with the pattern x:_.

Upvotes: 3

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