On 2005-04-20 19:04:32 -0700, "Alexandre Weffort Thenorio" <alethenorio@home.se> said:
As usual a beginner in Haskell. Trying to write a simple program in haskel shown below
[snip] getLeft :: String -> String -> String getRight :: String ->String -> String
outputLine keyno key orgFile = do part1 <- getLeft keyno orgFile part2 <- getRight keyno orgFile total <- part1 ++ (strUpper key) ++ part2 ++ "\n"
[snip] And I keep getting the error
changecode.hs:42: Couldn't match `[a]' against `Char' Expected type: [a] Inferred type: Char In the first argument of `(++)', namely `part1' In a 'do' expression: total <- part1 ++ ((strUpper key) ++ (part2 ++ "\n"))
You should be using: let part1 = getLeft keyno orgFile let part2 = getRight keyno orgFile let total = part1 ++ (strUpper key) ++ part2 ++ "\n" The problem is that the "part1 <- ..." syntax is for extracting the result from a monadic computation. When you read from a file like "hexFile <- readFile "file"", readFile is a computation in the IO monad, and you extract hexFile from the monad. The list [] type is also a monad, and String is really [Char], so "part1 <- getLeft keyno orgFile" implies that part1 is of type Char, which is a single element extracted from the list of Chars returned by the monadic computation (in the [] monad) "getLeft keyno orgFile". That leads to the error you see. part1's inferred type is Char, and the ++ function expects a list of some type ([a]), which Char is obviously not. The "let" syntax binds a variable instead of extracting it from a monadic computation, which is what you want for these three lines. Hope that helps! -- Peter Davis <pediddle@pediddle.net> "Furthermore, I believe bacon prevents hair loss!"