Haskell - how to write array to file?

Question

I am trying:

saveArr = do
    outh <- openFile "test.txt" WriteMode
    hPutStrLn outh [1,2,3]
    hClose outh

but it doesn't works... output:

No instance for (Num Char) arising from the literal `1'

EDIT OK hPrint works with ints but what about float number in array? [1.0, 2.0, 3.0]?

Answer 1

hPutStrLn can only print strings. Perhaps you want hPrint?

hPrint outh [1,2,3]

Answer 2

Arrays, Lists and Strings exists only in imagination of programmer and as a term in some languages.

File is a sequence of bytes, so when you want to write something to it you should encode that imaginary String/List/Array into sequence of bytes (by show or by something from Storable etc).
As well terminal is a sequence of bytes which is encoded representation of actions needed to show something to user.

You have many ways to encode. You can make CSV representation of array by foldr (\a b -> a (',' : b)) "\n" (map shows [1,2,3]) or you may want to print it show [1,2,3]

Answer 3

derive Binary for your type, then write the data in binnary form using 'encodeFile' from the Data.Binary package. This is similar to writing the data out as a bytestring.