Re: [Haskell-cafe] Avoiding boilerplate retrieving GetOpt cmd line args

27 Jul 2007

      On Friday 27 July 2007, Dave Bayer wrote:
...
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?
What you want can (almost) be done as follows:

{-# OPTIONS_GHC -fglasgow-exts #-}
import Data.Generics
import Data.Typeable

data Flag
  = Filter String
  | DateFormat String
  | DocStart String
  | DocEnd String
  deriving (Typeable, Data)

getString :: Flag -> String -> Flag -> String
getString c df f | toConstr c /= toConstr f = df
getString c df (Filter s) = s
getString c df (DateFormat s) = s
getString c df (DocStart s) = s
getString c df (DocEnd s) = s

This version uses overlapping patterns, of course; it should be evident how to 
change that if you want.

Call it as

getString Filter{} "Markdown.pl" flag

Jonathan Cast
http://sourceforge.net/projects/fid-core
http://sourceforge.net/projects/fid-emacs