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Checking for correct invocation of a command line haskell program

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thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>./args.sh
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[[Category:How to]]
This demonstrates checking for correct invocation of a command line / shell haskell program
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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
 
   
**********
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==Usage examples==
runghc arghandling-nice.hs 0 1 2
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"0 1 2"
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<code>
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runghc arghandling-nice.hs 0 1 2
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"0 1 2"
   
  +
runghc arghandling-ugly.hs 0 1 2
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"0 1 2"
   
runghc arghandling-ugly.hs 0 1 2
 
"0 1 2"
 
   
  +
runghc arghandling-nice.hs 0 1
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*** Exception: args length does not equal 3. args: : ["0","1"]
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usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg
   
runghc arghandling-nice.hs 0 1
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runghc arghandling-ugly.hs 0 1
*** Exception: args length does not equal 3. args: : ["0","1"]
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*** 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
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</code>
   
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==The code==
   
runghc arghandling-ugly.hs 0 1
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arghandling-nice.hs:
*** Exception: user error (Pattern match failure in do expression at arghandling-ugly.hs:3:10-29)
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<haskell>
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import System
   
thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-nice.hs
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main = do args <- getArgs
import System
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let usagemsg = "usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg"
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case args of
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[first,second,third] -> process first second third
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_ -> error $ "args length does not equal 3. args: : " ++ ( show args ) ++ "\n" ++ usagemsg
   
main = do args <- getArgs
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process a b c = print $ unwords [a,b,c]
let usagemsg = "usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg"
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</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]
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arghandling-ugly.hs
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<haskell>
  +
import System
   
thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-ugly.hs
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main = do [first,second,third] <- getArgs
import System
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process first second third
   
main = do [first,second,third] <- getArgs
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process a b c = print $ unwords [a,b,c]
process first second third
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</haskell>
   
process a b c = print $ unwords [a,b,c]
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==See also:==
 
 
See also:
 
   
 
* [[Simple unix tools]]
 
* [[Simple unix tools]]
  +
* [[Error reporting strategies]]
 
* [http://leiffrenzel.de/papers/commandline-options-in-haskell.html Command line options in haskell]
 
* [http://leiffrenzel.de/papers/commandline-options-in-haskell.html Command line options in haskell]
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* [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: