Difference between revisions of "FP Laboratory 12"
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== Working with files - monad IO == | == Working with files - monad IO == | ||
| − | * Create a program that reads a file and number it's lines. | + | * Create a program that reads a file and number it's lines and writes it on the screen. |
<div class="mw-collapsible mw-collapsed" data-collapsetext="Hide solution" data-expandtext="Show solution"> | <div class="mw-collapsible mw-collapsed" data-collapsetext="Hide solution" data-expandtext="Show solution"> | ||
<syntaxhighlight lang="Haskell"> | <syntaxhighlight lang="Haskell"> | ||
| + | import System.IO | ||
| + | import Control.Exception | ||
| + | |||
| + | main = do fromHandle <- opf "Read from: " ReadMode | ||
| + | contents <- hGetContents fromHandle | ||
| + | putStr (numberLines contents) | ||
| + | hClose fromHandle | ||
| + | |||
| + | opf :: String -> IOMode -> IO Handle | ||
| + | opf prompt mode = do putStr prompt | ||
| + | name <- getLine | ||
| + | catch (openFile name mode ) | ||
| + | (\e -> do putStr ("Can't open "++name++":"++show (e :: IOException) ++"\n") | ||
| + | opf prompt mode) | ||
| + | |||
| + | numberLines::String -> String | ||
| + | numberLines text = let l = lines text | ||
| + | numbered = zip [1..] l | ||
| + | result = [show x ++".\t"++content++"\n" | (x, content)<-numbered] | ||
| + | in concat result | ||
</syntaxhighlight> | </syntaxhighlight> | ||
</div> | </div> | ||
<div style="clear:both"></div> | <div style="clear:both"></div> | ||
Revision as of 10:42, 24 September 2020
Working with files - monad IO
- Create a program that reads a file and number it's lines and writes it on the screen.
import System.IO
import Control.Exception
main = do fromHandle <- opf "Read from: " ReadMode
contents <- hGetContents fromHandle
putStr (numberLines contents)
hClose fromHandle
opf :: String -> IOMode -> IO Handle
opf prompt mode = do putStr prompt
name <- getLine
catch (openFile name mode )
(\e -> do putStr ("Can't open "++name++":"++show (e :: IOException) ++"\n")
opf prompt mode)
numberLines::String -> String
numberLines text = let l = lines text
numbered = zip [1..] l
result = [show x ++".\t"++content++"\n" | (x, content)<-numbered]
in concat result
