Real World Haskell ch4 pp71-72 >> do x <- foo >> y <- bar >> >> the y <- bar must be directly under the x on the previous line or it's a >> syntax error, and the error you get from GHC is "the last statement of a do >> construct must be an expression" > > Huh, so this Haskell syntax actually prevents you from indenting something that would be indented in any other language. Okay...
I didn't read it like this. The x and y must be lined up because they are both part of the same set of do-statements. If the y line was a standalone statement, then it could be indented as shown in this code segment. But in that case there's no need for a do, since the point of do is to order statements in sequential time. the following is a complete Haskell program, using do, which passes through its input file to an output file. The > specifies code lines, other lines are considered comments. (I hope gmail preserves the spacing.) Real World Haskell ch4 pp71-72 > import System.Environment (getArgs) > > interactWith function inputFile outputFile = do > input <- readFile inputFile > writeFile outputFile (function input) > > main = mainWith myFunction > where mainWith function = do > args <- getArgs > case args of > [input,output] -> interactWith function input output > _ -> putStrLn "error: need exactly two arguments" Replace id with any function (of the same type) to process the data. > myFunction = id
