Difference between revisions of "Checking for correct invocation of a command line haskell program"
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| − | Running our example: |
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thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>./args.sh |
thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>./args.sh |
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| − | + | This demonstrates checking for correct invocation of a command line / shell haskell program |
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| − | We run a nice example, |
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********** |
********** |
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| − | nice arg handling, no errors: |
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"0 1 2" |
"0 1 2" |
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| − | ugly arg handling, no errors: |
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"0 1 2" |
"0 1 2" |
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| − | nice arg handling, errors: |
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*** Exception: args length does not equal 3. args: : ["0","1"] |
*** Exception: args length does not equal 3. args: : ["0","1"] |
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usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg |
usage example: $ runghc arghandling-nice.hs firstarg secondarg thirdarg |
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| − | ugly arg handling, errors: |
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*** Exception: user error (Pattern match failure in do expression at arghandling-ugly.hs:3:10-29) |
*** Exception: user error (Pattern match failure in do expression at arghandling-ugly.hs:3:10-29) |
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| − | Source code... |
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| − | args.sh: |
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| − | thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat args.sh |
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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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| − | ********** |
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| − | EOF |
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| − | echo nice arg handling, no errors: |
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| − | echo |
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| − | echo |
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| − | echo ugly arg handling, no errors: |
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| − | echo |
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| − | echo |
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| − | echo nice arg handling, errors: |
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| − | echo |
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| − | echo |
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| − | echo ugly arg handling, errors: |
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thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-nice.hs |
thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-nice.hs |
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process a b c = print $ unwords [a,b,c] |
process a b c = print $ unwords [a,b,c] |
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thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-ugly.hs |
thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-ugly.hs |
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Revision as of 14:02, 10 April 2007
thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>./args.sh This demonstrates checking for correct invocation of a command line / shell haskell program 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
********** 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 *** 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)
thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat 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]
thartman@linodehaskell:~/learning/haskell/UnixTools/arghandling>cat arghandling-ugly.hs import System
main = do [first,second,third] <- getArgs
process first second third
process a b c = print $ unwords [a,b,c]