Haskell Error - Couldn't match expected type

Question

i want to write a function for building a list of random numbers and here comes the code ive written

buildlist :: Int -> Int -> [Int]
buildlist n m = do
    seed <- getStdGen
    let l = randomRs (0, m) seed
    let list = take n l
    return list

and then the errors

    Couldn't match expected type `[t0]' with actual type `IO StdGen'
In a stmt of a 'do' block: seed <- getStdGen
In the expression:
  do { seed <- getStdGen;
       let l = randomRs ... seed;
       let list = take n l;
       return list }
In an equation for `buildlist':
    buildlist n m
      = do { seed <- getStdGen;
             let l = ...;
             let list = ...;
             .... }

ps.haskell is so different from c,java,ruby that i feel i've nerver learnt coding

Koterpillar · Accepted Answer · 2013-04-16 09:50:03Z

6

Because you are using IO (getStdGen), the whole function must be in IO monad. Change the return type to

buildList :: Int -> Int -> IO [Int]

and do read a good book :-)

answered Apr 16, 2013 at 9:50

Koterpillar

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2 Comments

yck Over a year ago

thanks a lot.it's hard to buy a book about haskell in china. and english website is hard to understand. T_T

mhwombat Over a year ago

There are two excellent books that you can read online: book.realworldhaskell.org/read and learnyouahaskell.com. Unfortunately, I'm not aware of any materials in Chinese, but if you speak any other languages, check haskell.org/haskellwiki/Books and haskell.org/haskellwiki/Tutorials. Translations of tutorials in other languages do appear from time to time, so keep checking those sites. And perhaps you'll translate one of the tutorials into your language!

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