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Checking for correct invocation of a command line haskell program

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Running our example:
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[[Category:How to]]
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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
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==Usage examples==
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<code>
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runghc arghandling-nice.hs 0 1 2
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"0 1 2"
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thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>./args.sh
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runghc arghandling-ugly.hs 0 1 2
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Checking for correct invocation of a command line / shell haskell program
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"0 1 2"
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We run a nice example,
 
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alongside example of "ugly" argument checking that is simpler to write,
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runghc arghandling-nice.hs 0 1
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but gives a less user friendly error message when the program is invoked incorrectly
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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
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runghc arghandling-ugly.hs 0 1
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*** Exception: user error (Pattern match failure in do expression at arghandling-ugly.hs:3:10-29)
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</code>
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**********
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==The code==
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nice arg handling, no errors:
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"0 1 2"
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arghandling-nice.hs:
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<haskell>
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import System
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ugly arg handling, no errors:
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main = do args <- getArgs
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"0 1 2"
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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
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process a b c = print $ unwords [a,b,c]
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</haskell>
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nice arg handling, errors:
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arghandling-ugly.hs
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*** Exception: args length does not equal 3. args: : ["0","1"]
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<haskell>
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usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg
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import System
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main = do [first,second,third] <- getArgs
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process first second third
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ugly arg handling, errors:
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process a b c = print $ unwords [a,b,c]
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*** 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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Source code...
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==See also:==
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args.sh:
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* [[Simple unix tools]]
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* [[Error reporting strategies]]
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* [http://leiffrenzel.de/papers/commandline-options-in-haskell.html Command line options in haskell]
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thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat args.sh
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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)]
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cat <<EOF
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Checking for correct invocation of a command line / shell haskell program
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We run a nice example,
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alongside example of "ugly" argument checking that is simpler to write,
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but gives a less user friendly error message when the program is invoked incorrectly
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**********
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EOF
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echo nice arg handling, no errors:
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runghc arghandling-nice.hs 0 1 2
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echo
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echo
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echo ugly arg handling, no errors:
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runghc arghandling-ugly.hs 0 1 2
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echo
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echo
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echo nice arg handling, errors:
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runghc arghandling-nice.hs 0 1
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echo
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echo
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echo ugly arg handling, errors:
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runghc arghandling-ugly.hs 0 1
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thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-nice.hs
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import System
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main = do args <- getArgs
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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
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process a b c = print $ unwords [a,b,c]
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thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-ugly.hs
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import System
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main = do [first,second,third] <- getArgs
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process first second third
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process a b c = print $ unwords [a,b,c]
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Current revision

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

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)

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]

3 See also:

Retrieved from "http://www.haskell.org/haskellwiki/Checking_for_correct_invocation_of_a_command_line_haskell_program"

Category: How to