[Haskell-cafe] extending and reusing cmdargs option specs ?

wren ng thornton wren at freegeek.org
Tue Aug 9 04:08:57 CEST 2011

On 8/8/11 1:59 PM, Simon Michael wrote:
> And, I may have made things worse for myself in the reuse/avoiding
> boilerplate department: I'm not sure how to reuse a cmdargs options data
> structure, extending it with a few more options.

This is a big problem I'm dealing with too lately. In particular, what I 
would like is if cmdargs could do flattening of structures, either 
automatically or with a new annotation combinator. With this it would be 
easy to have a basic FooBar program:

     data FooBar = FooBar
         { foo :: Foo
         , bar :: Bar
         deriving (Data, Typeable,...)

     getFooBarMode :: IO (Mode (CmdArgs FooBar))
     getFooBarMode = ...

and then extend it to a FooBarBaz program:

     data FooBarBaz = FooBarBaz
         { fooBar :: FooBar
         , baz :: Baz
         deriving (Data, Typeable,...)

     getFooBarBazMode :: IO (Mode (CmdArgs FooBarBaz))
     getFooBarBazMode = ...

The big trick is that I should be able to call getFooBarMode from 
getFooBarBazMode equivalently to if I had defined,

     data FooBarBaz = FooBarBaz
         { foo :: Foo
         , bar :: Bar
         , baz :: Baz
         deriving (Data, Typeable,...)

and copied over getFooBarMode inline into getFooBarBazMode. The problem 
is that the bulk of the code is in the Mode-generating functions, and 
it's also the sort of code that's hardest to keep in sync 

I don't mind the extra indirection in the Mode structures since I don't 
actually use them in the program itself; I have an internal 
configuration type that I compile the FooBarBaz down to, so that I can 
perform additional semantic consistency checks before execution, as well 
as precomputing what the commandline arguments dereference into (e.g., 

> As always, thanks a lot for cmdargs!
> -Simon

Indeed. Aside from this feature request I've been quite pleased :)

Live well,

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