[Haskell-cafe] Over general types are too easy to make.

Alexander Solla alex.solla at gmail.com
Sun Sep 2 19:06:04 CEST 2012

On Sun, Sep 2, 2012 at 9:40 AM, <timothyhobbs at seznam.cz> wrote:

> The thing is, that one ALWAYS wants to create a union of types, and not
> merely an ad-hock list of data declarations.  So why does it take more code
> to do "the right thing(tm)" than to do "the wrong thing(r)"?

Because a union type is a complex union of parts, and the parts need to be
deconstructed in order to be acted upon.  There is not a unique way to do
this -- different "unwrappings" have different properties and must match
your use case.

Perhaps you should read "Data types ala carte" (W. Swiestra) [1], which
provides an approach to constructing "open" data types (i.e., sum types to
which new summands can be added)

[1] http://www.cs.ru.nl/~W.Swierstra/Publications/DataTypesALaCarte.pdf
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