Re: [Haskell-cafe] Avoiding boilerplate retrieving GetOpt cmd line args
On Thu, Jul 26, 2007 at 10:25:06PM -0700, 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
I don't know if my reply is going to be helpful to you. This is what I
would suggest: why don't you create a data type with label records,
and than you store that data type in a IORef and update the IORef. At
the end you just read the IORef with your updated data:
data Config = { filter :: String
, dateFormat :: String
, etc ...
}
Then you create a new IOref with config, and, with getOpt, you update
the IOref with modifyIORef. At the very end you read the modified
IOref.
This way you can have default options to be modified with command line
options.
I don't know if it is clear, but I adopted this approach in a program
I'm writing. A program with 14 command line options.
Have a look at this part here (starting from data Opts=):
http://gorgias.mine.nu/repos/xmobar/Main.hs
This way I can load a configuration file and change some of the
options, configured in that file, with the given command line options.
I hope this is going to help you.
Andrea
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Re: [Haskell-cafe] Avoiding boilerplate retrieving GetOpt cmd line args
Hi Why not: data Flag = Filter String | DateFormat String | DocStart String | DocEnd String Becomes: data Flag = Flag Key String data Key = Filter | DateFormat | DocStart | DocEnd getString :: Flag - Key - String getString (Flag x y) key = if key == x then y else You can easily extend this to defaults: defaults = [(DocStart,1)] then lookup, instead of just as the else clause. If you have to use Data/Typeable you will no longer be writing portable Haskell, and while they are great, they aren't the thing to use here. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Avoiding boilerplate retrieving GetOpt cmd line args
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
pgp7JvE0fRdTA.pgp
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[Haskell-cafe] Avoiding boilerplate retrieving GetOpt cmd line args
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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Re: [Haskell-cafe] Avoiding boilerplate retrieving GetOpt cmd line args
Hi,
Not sure if this will help avoid the boilerplate, but I've always liked
the approach at
http://leiffrenzel.de/papers/commandline-options-in-haskell.html
(particularly the section Towards a higher level) for being able to
specify defaults. It's the best resource I've found on command line
options in Haskell so far.
Levi
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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