[Haskell-beginners] Record types with multiple constructors

[Magnus Therning]

On Sun, Dec 07, 2014 at 09:37:11AM +0000, Derek McLoughlin wrote:
> Hi,
> 
> Record types usually have a single constructor. I've even seen blog
> posts that suggest that they must have a single constructor. However,
> multiple constructors are allowed:
> 
> data Employee = RegularEmployee {
>                                   name :: String
>                              } |
>                              Supervisor {
>                                   name :: String,
>                                   salesTarget :: Double
>                              }
>                              Manager {
>                                   name :: String,
>                                   salesTarget :: Double
>                                   budget :: Double
>                              }
> 
> I don't see this used much in Haskell code - either in explanatory
> books/tutorials or in code I've examined on GitHub. Are there
> drawbacks to using multiple constructors in this way?

I think you might see it fairly frequently in users of command line
parser libs, e.g. optparse-applicative[^1], for accepting
sub-commands.

/M

[^1]: http://hackage.haskell.org/package/optparse-applicative

-- 
Magnus Therning                      OpenPGP: 0xAB4DFBA4 
email: magnus at therning.org   jabber: magnus at therning.org
twitter: magthe               http://therning.org/magnus

Perl is another example of filling a tiny, short-term need, and then
being a real problem in the longer term.
     -- Alan Kay
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