71 lines
2.1 KiB
XML
71 lines
2.1 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
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<pkgmetadata>
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<herd>haskell</herd>
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<maintainer>
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<email>haskell@gentoo.org</email>
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</maintainer>
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<longdescription>
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ReadArgs provides the @readArgs@ IO action, which lets you tell the compiler
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to parse the command line arguments to fit the type signature you give.
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For example @(a :: Int, b :: String, c :: Float) <- readArgs@ would
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parse the first runtime argument as an @Int@, the second as a @String@ (no
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quotes required) and the third as a @Float@.
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If the runtime arguments are incompatible with the type signature,
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then a simple usage statement is given of the types needed.
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Continuing the previous example, if it was used in a
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program named @Example@, the error message for the above
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action would be:
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@
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usage: Example Int String Float
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@
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Any type that has both @Typeable@ and @Read@ instances
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can be used. @Char@, @String@, and @Text@ are handled specially so that
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command line arguments for both do not require quotes (as their
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@Read@ instances do). A special instance is provided for @FilePath@ so
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that no constructor or quotes are required.
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@readArgs@ also supports optional arguments and variadic arguments.
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Optional arguments are specified using @Maybe@, and variadic arguments
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using a list. @(a :: Int, b :: Maybe String, c :: [Float]) <- readArgs@
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would successfully parse any of the following sets of command line arguments:
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@
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Example 1
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Example 1 2 3 4
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Example 1 foo
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Example 1 foo 2 3 4
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@
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But not
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@
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Example
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Example foo
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Example 1.0
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@
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Usage statements for optional and variadic arguments use command-line
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parlance:
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@
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usage: Example Int [String] [Float..]
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@
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Note that both optional and variadic parsers are greedy by default
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(so @Example 1 2 3 4@ was parsed as @(1, "2", [3.0,4.0])@. They
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may both be made non-greedy through use of the @NonGreedy@ constructor:
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@
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( a :: Int
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, NonGreedy b :: NonGreedy Maybe String
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, NonGreedy c :: NonGreedy [] Float
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) <- readArgs
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@
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</longdescription>
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</pkgmetadata>
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