[Haskell-cafe] CmdArgs non-flag arguments with newtypes

Michael Orlitzky michael at orlitzky.com
Mon Jul 29 17:04:25 CEST 2013

On 07/23/2013 11:48 PM, wren ng thornton wrote:
> On 7/23/13 9:01 PM, Michael Orlitzky wrote:
>> Obviously not what I want! Has anyone
>> else run into this? Figured out a workaround?
> I haven't run into this specific problem, but I do have a sort of
> workaround. Whenever dealing with CmdArgs (or any similar system) I
> typically define *two* record types.
> The first one just gets the raw input from CmdArgs; no more, no less.
> Thus, for your issue, this record would use String. For other issues
> mentioned recently about optionality, this record would use Maybe.
> The second one is the one actually used by the program as the
> configuration object. This one is generated from the first by performing
> various sanity checks, filling in defaults, converting types from their
> CmdArgs version to the version I actually want, etc.
> IME, regardless of language, trying to conflate these notions of an
> external-facing parser-state and an internal-facing configuration-record
> just causes problems and accidental complexity. It's best to keep them
> separate IMO.

It's not the internal configuration and command-line arguments that I
wanted to share, but rather the config /file/ and command-line
arguments. Those two are then mashed together into the real
configuration data structure at run time.

The reason for the newtype is for the config file parsing -- without the
newtype, I'm forced to create orphan instances like some kind of animal =)

In any case, this turned out to be a real bug and not the usual user
error. Neil was nice enough to fix it, and it looks like it already hit
hackage, so I'm gonna go give it a shot!

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