Newbie question
Hello! I am learning Haskell according to the Yet Another Haskell Tutorial by Hal Daume Ill. One of the exercises involves a) asking the user to enter several numbers (while the end of the sequence is indicated by entering 0) b) calculate the sum of those numbers. The program given below tries to do that. I can read a list of strings. Then, I try to convert the list of strings into a list of numbers by means of map. That is, given a list [1,2,3], I want to convert it into [1,2,3]. In GHCi, this is done via map read [1,2,3] However, when I apply this same call in my program below, I'm getting following error when loading the program in GHCi: error-message Loading package base ... linking ... done. Compiling Main ( AskForNumbers.hs, interpreted ) AskForNumbers.hs:10: Couldn't match `IO' against `[]' Expected type: IO t Inferred type: [b] In the application `map read words' In a 'do' expression: map read words Failed, modules loaded: none. /error-message Line 10 corresponds to the statement map read words, which seems to work in GHCi. What am I doing wrongly? Thanks in advance Dmitri Pissarenko PS: What follows is my Haskell program. module Main where import IO main = do hSetBuffering stdin LineBuffering words - askForNumbers printWords words map read words putStrLn The sum is foldl (+) 0 words askForNumbers = do putStrLn Please enter a number: text - getLine if text == then return [] else if text == 0 then return [] else do rest - askForNumbers return (text : rest) printWords [] = putStrLn EOL printWords (h : t) = do putStrLn h printWords t ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Newbie question
Dmitri Pissarenko [EMAIL PROTECTED] writes: a) asking the user to enter several numbers (while the end of the sequence is indicated by entering 0) b) calculate the sum of those numbers. ... Here is a corrected version: module Main where import IO Delete this. main = do hSetBuffering stdin LineBuffering This is extraneous - stdin should be line buffered by default. words - askForNumbers printWords words map read words putStrLn The sum is foldl (+) 0 words Here you map read over words and discard the result. Then you wold over the words and produce an interger, whereas you should produce an IO value from the do block. Here is a complete implementation (untested): main = do words - askForNumbers mapM_ putStrLn words putStrLn EOL let nums = map read words print nums putStrLn The sum is print (foldl (+) 0 nums) - Einar Karttunen ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Newbie question
comments inline... module Main where import IO main = do hSetBuffering stdin LineBuffering words - askForNumbers printWords words map read words putStrLn The sum is foldl (+) 0 words as you noted map read words is a problematic line. The problem that GHC tells you about is that map read words creates a value of type (Read a) = [a], where but each line in an IO do block needs to have type IO a. THe foldl (+) 0 words line has the same problem; it's not an IO value. A trick you can use is to use let to bind a pure value inside a do block, see below. I seems to me that you are thinking that map read will alter words from a list of Strings to a list of Ints. This is a fundamental no-no in a pure functional language like Haskell. What you want is to create a new list of integers from your words list. The following changes makes your program work. main = do hSetBuffering stdin LineBuffering words - askForNumbers printWords words --map read words let ints = map read words putStrLn The sum is --foldl (+) 0 words putStrLn (show (foldl (+) 0 ints)) askForNumbers = do putStrLn Please enter a number: text - getLine if text == then return [] else if text == 0 then return [] else do rest - askForNumbers return (text : rest) This is pretty verbose and (IMO) hard to read. I'd probably write it this way: askForNumbers = do putStrLn Pleas enter a number: text - getLine case text of - return [] 0 - return [] _ - askForNumbers = return . (text:) printWords [] = putStrLn EOL printWords (h : t) = do putStrLn h printWords t This could also be more succinct with sequence_, although increased redability is arguable. printWords x = sequence_ (map putStrLn x) putStrLn EOL ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Newbie question
Thanks all for the help! ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
RE: (Stupid?) newbie question
Hi, not a stupid question, just a stupid bug in 4.03 ;-) This has been fixed in the patch available from the ghc-win32 web page, http://www.dcs.gla.ac.uk/~sof/ghc-win32.html http://www.dcs.gla.ac.uk/~sof/ghc-win32.html hth --sigbjorn -Original Message- From: felix [mailto:[EMAIL PROTECTED]] Sent: Monday, August 16, 1999 00:45 To: glasgow-haskell-users Subject: (Stupid?) newbie question Hi, folks! I have recently downloaded ghc-4.03 (under win32/cygwin) and everything seems to work great, but somehow the following code snippet does not work as intended: import System main = do argv - getArg print argv This does print "[]" - no arguments! Since the same thing fails in Hugs too, I must have made some mistake. please enlighted me. thanks, felix
