I'm currently working through "Yet Another Haskell Tutorial" by H. Daume and have a question on how to proceed with his exercise 3.10. The exercise question is: Write a program that will repeatedly ask the user for numbers until he types in zero, at which point it will tell him the sum of all the numbers, the product of all the numbers, and, for each number, its factorial. The hint that is given is to write an IO action that reads a number and either returns an empty list (if the number is zero) or recurses itself making a list from the number and the result of the recursive call. Daume presents and example of such a function: askForWords = do putStrLn "Please enter a word:" word <- getLine if word == "" then return [] else do rest <- askForWords return (word : rest) so, based on his example I created the function: getNums = do putStrLn "Enter a number (zero to stop):"; strNum <- getLine let num = read strNum if num == 0 then return [] else do rest <- getNums return (num:rest) loading the file that contains the above function into GHCi (version 6.8.3) gives the following error "The last statement in a 'do' construct must be an expression" being a beginner, I'm at a loss at the cause of the error and how to fix it -- any suggestions? thanks, george
It compiles fine here with GHC 6.8.2:
*Main> getNums
Enter a number (zero to stop):
4
Enter a number (zero to stop):
3
Enter a number (zero to stop):
0
[4,3]
Are you sure the problem is in this function?
Fernando Henrique Sanches
On Tue, Jun 9, 2009 at 1:05 AM, George Huber
I'm currently working through "Yet Another Haskell Tutorial" by H. Daume and have a question on how to proceed with his exercise 3.10.
The exercise question is: Write a program that will repeatedly ask the user for numbers until he types in zero, at which point it will tell him the sum of all the numbers, the product of all the numbers, and, for each number, its factorial.
The hint that is given is to write an IO action that reads a number and either returns an empty list (if the number is zero) or recurses itself making a list from the number and the result of the recursive call.
Daume presents and example of such a function:
askForWords = do putStrLn "Please enter a word:" word <- getLine if word == "" then return [] else do rest <- askForWords return (word : rest)
so, based on his example I created the function:
getNums = do putStrLn "Enter a number (zero to stop):"; strNum <- getLine let num = read strNum if num == 0 then return [] else do rest <- getNums return (num:rest)
loading the file that contains the above function into GHCi (version 6.8.3) gives the following error "The last statement in a 'do' construct must be an expression"
being a beginner, I'm at a loss at the cause of the error and how to fix it -- any suggestions?
thanks, george
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On Tue, Jun 09, 2009 at 12:05:21AM -0400, George Huber wrote:
loading the file that contains the above function into GHCi (version 6.8.3) gives the following error "The last statement in a 'do' construct must be an expression"
This error can also sometimes be triggered by indentation problems. Check to make sure you are using only spaces and not tabs to indent. (Tabs are interpreted as a certain number of spaces by ghc---I forget how many---which may be different than the number of spaces your editor is configured to show them as, which can lead to indentation being screwy. Good Haskell practice is to never use tabs; in practice most editors have an option to turn tabs into spaces.) Dunno if that's really your problem but it's worth a shot! -Brent
loading the file that contains the above function into GHCi (version 6.8.3) gives the following error "The last statement in a 'do' construct must be an expression"
being a beginner, I'm at a loss at the cause of the error and how to fix it -- any suggestions?
thanks, george
No problem here too, ghc 6.8.2. Maybe seeing how things are when layout style is not used helps you see a possible layout problem. Maybe it doesn't, and you can just forget the example below :) module Main (main) where { getNums = putStrLn "Enter a number (zero to stop):" >> do { num <- readLn ; if num == 0 then return [] else do {rest <- getNums ; return (num:rest)} } ; main = getNums >>= putStrLn . show } Best, Maurício
participants (4)
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Brent Yorgey -
Fernando Henrique Sanches -
George Huber -
Maurício