On Sep 18, 2011, at 11:28 AM, Brandon Allbery <allber...@gmail.com> wrote:
> On Sat, Sep 17, 2011 at 22:11, Anton Tayanovskyy
> <anton.tayanovs...@gmail.com> wrote:
> By the way, can Haskell have a type that admits regular expression and
> only those? I mostly do ML these days, so trying to write up a regex
> types in Haskell I was unpleasantly surprised to discover that there
> are all sorts of exotic terms inhabiting it, which I did not have to
> worry about in ML. Besides `undefined` you can have for terms that try
> to define a grammar with nested parentheses. Using regex-applicative
> syntax:
>
> expr = ... <|> pure (\ _ x _ -> x) <*> sym "(" <*> expr <*> sym ")"
>
> The general case for this is solving the Halting Problem, so neither Haskell
> nor any other language can do it. You can disallow infinite values by
> encoding the length into the type, but (a) in Haskell this is painful and (b)
> runtime values now require runtime types, which you can accommodate but at
> the price of reintroducing the problems you are trying to prevent.
> (Dependently typed languages work better for this, but I suspect the result
> is rather more draconian than you intend.)
You needn't solve the halting problem. Any reasonable total language can
prevent this case and there is no need to keep types as simple as these around
at runtime. See the Agda total parser combinator library for an example, but
yes, dependent types are overkill.
I usually enforce constraints like this with ! patterns in the constructors,
which lets me enforce the fact that at least I know that any attempt to define
a cycle like this will bottom out, so I can safely think only inductive
thoughts from there out.
Another way is to just define a template Haskell regex quasi quoter, and
enforce the limitation there. That of course costs you runtime generation.
-Edward Kmett_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
