Personal tools

Checking for correct invocation of a command line haskell program

From HaskellWiki

(Difference between revisions)
Jump to: navigation, search
m (Category)
 
(9 intermediate revisions by 2 users not shown)
Line 1: Line 1:
    thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>./args.sh
+
[[Category:How to]]
    This demonstrates checking for correct invocation of a command line / shell haskell program
+
This page shows checking for correct invocation of a command line / shell haskell program.  The first example is  a version which gives a "user friendly" error message when invoked incorrectly, the second is an example of "ugly" argument checking that is slightly simpler to write, but gives a less user friendly error message
    It runs a version which gives a "user friendly" error message when invoked incorrectly,
+
    alongside an example of "ugly" argument checking that is slightly simpler to write,
+
    but gives a less user friendly error message
+
  
    **********
+
==Usage examples==
    runghc arghandling-nice.hs 0 1 2
+
 
    "0 1 2"
+
<code>
 +
runghc arghandling-nice.hs 0 1 2
 +
"0 1 2"
  
 +
runghc arghandling-ugly.hs 0 1 2
 +
"0 1 2"
  
    runghc arghandling-ugly.hs 0 1 2
 
    "0 1 2"
 
  
 +
runghc arghandling-nice.hs 0 1
 +
*** Exception: args length does not equal 3. args: : ["0","1"]
 +
usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg
  
    runghc arghandling-ugly.hs 0 1
+
runghc arghandling-ugly.hs 0 1
    *** Exception: args length does not equal 3. args: : ["0","1"]
+
*** Exception: user error (Pattern match failure in do expression at arghandling-ugly.hs:3:10-29)
    usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg
+
</code>
  
 +
==The code==
  
    runghc arghandling-ugly.hs 0 1
+
arghandling-nice.hs:
    *** Exception: user error (Pattern match failure in do expression at arghandling-ugly.hs:3:10-29)
+
<haskell>
 +
import System
  
    thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-nice.hs
+
main = do args <- getArgs
    import System
+
          let usagemsg = "usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg"
 +
          case args of
 +
            [first,second,third] -> process first second third
 +
            _                    -> error $ "args length does not equal 3. args: : " ++ ( show args ) ++ "\n" ++ usagemsg
  
    main = do args <- getArgs
+
process a b c = print $ unwords [a,b,c]
              let usagemsg = "usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg"
+
</haskell>
              case args of
+
                [first,second,third] -> process first second third
+
                _                    -> error $ "args length does not equal 3. args: : " ++ ( show args ) ++ "\n" ++ usagemsg
+
  
    process a b c = print $ unwords [a,b,c]
+
arghandling-ugly.hs
 +
<haskell>
 +
import System
  
    thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-ugly.hs
+
main = do [first,second,third] <- getArgs
    import System
+
          process first second third
  
    main = do [first,second,third] <- getArgs
+
process a b c = print $ unwords [a,b,c]
              process first second third
+
</haskell>
  
    process a b c = print $ unwords [a,b,c]
+
==See also:==
 +
 
 +
* [[Simple unix tools]]
 +
* [[Error reporting strategies]]
 +
* [http://leiffrenzel.de/papers/commandline-options-in-haskell.html Command line options in haskell]
 +
* [http://groups.google.de/group/fa.haskell/browse_thread/thread/6359767a8e64bc5d/e5286a2783c22bbe?lnk=st&q=haskell+string+fill+ins&rnum=1&hl=en#e5286a2783c22bbe string fill-ins (discussion at haskell cafe)]

Latest revision as of 21:23, 15 June 2007

This page shows checking for correct invocation of a command line / shell haskell program. The first example is a version which gives a "user friendly" error message when invoked incorrectly, the second is an example of "ugly" argument checking that is slightly simpler to write, but gives a less user friendly error message

[edit] 1 Usage examples

runghc arghandling-nice.hs 0 1 2 "0 1 2"

runghc arghandling-ugly.hs 0 1 2 "0 1 2"


runghc arghandling-nice.hs 0 1

*** Exception: args length does not equal 3. args: : ["0","1"]

usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg

runghc arghandling-ugly.hs 0 1

*** Exception: user error (Pattern match failure in do expression at arghandling-ugly.hs:3:10-29)

[edit] 2 The code

arghandling-nice.hs:

import System
 
main = do args <- getArgs
          let usagemsg = "usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg"
          case args of
            [first,second,third] -> process first second third
            _                    -> error $ "args length does not equal 3. args: : " ++ ( show args ) ++ "\n" ++ usagemsg
 
process a b c = print $ unwords [a,b,c]

arghandling-ugly.hs

import System
 
main = do [first,second,third] <- getArgs
          process first second third
 
process a b c = print $ unwords [a,b,c]

[edit] 3 See also: