Niklas Broberg wrote:
On Sun, Jun 21, 2009 at 9:24 PM, Andrew
Coppin<andrewcop...@btinternet.com> wrote:
data Foobar =
Foo Foobar |
Bar Foobar |
Zoo Foobar
I want the type system to track whether or not Zoo has been used in a
specific value. Sure, you can check for it at runtime, but I'd be happier if
the type system can guarantee its absence when required.
That's what GADTs are for:
data Flag = HasZoo | NoZoo
data Foobar a where
Foo :: Foobar a -> Foobar a
Bar :: Foobar a -> Foobar a
Zoo :: Foobar a -> Foobar HasZoo
...so I use "Foobar x" to mean any kind of value, "Foobar NoZoo" for a
value that definitely doesn't contain Zoo, and "Foobar HasZoo" for...
one that definitely does? Or maybe does? (Not that I care about this,
but for completeness I'd like to know.)
A slight complication is that Foobar refers to a second type which
exhibits a similar behaviour - but presumably I can utilise the same
solution there also...
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