Ok, I'm writing a command line tool, using System.Console.GetOpt to
handle command line arguments. My Flags structure so far is
data Flag
= Filter String
| DateFormat String
| DocStart String
| DocEnd String
...
and I want to write accessor functions that return the strings if
specified, otherwise returning a default. The best I've been able to
do is this:
getFilter = getString f "Markdown.pl"
where f (Filter s) = Just s
f _ = Nothing
getDateFormat = getString f "%B %e, %Y"
where f (DateFormat s) = Just s
f _ = Nothing
getDocStart = getString f "^{-$"
where f (DocStart s) = Just s
f _ = Nothing
getDocEnd = getString f "^-}$"
where f (DocEnd s) = Just s
f _ = Nothing
using a generic accessor function `getString`.
There are eight (and growing) needless lines here, where what I
really want to do is to pass the constructors `Filter`, `DateFormat`,
`DocStart`, or `DocEnd` to the function `getString`. ghci types each
of these as `String -> Flag`, so one at least knows how to type such
a `getString`, but using a constructor-passed-as-an-argument in a
pattern match is of course a "Parse error in pattern". (I expected as
much, but I had to try... `String -> Flag` is not enough information
to make it clear we're passing a constructor, rather than some hairy
arbitrary function, so such a pattern match would be undecidable in
general.)
So what's the right idiom for avoiding this boilerplate?
Thanks,
Dave
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