On 15 Sep 2010, at 16:29, Matias Eyzaguirre wrote: > Hi, > I'v been reading a small paper/lesson on writing parser combinators in > Haskell, and it seems more or less straightforward. In this case a parser is > defined thusly: >> type Parser a = String -> Maybe (a, String) > And then it goes on to list some simple parsers, and then starts going on > about combinators. > I was wondering how one would write quickcheck properties for the items > presented in the paper, and the answer seemed fairly straightforward for the > actual parsers. But how on earth would you write a test for a combinator? > Presumable one would need to make the type an instance of Arbitrary. > I see two problems: > Firstly, as far as i can tell, one cannot declare a type synonym to be an > instance of a type class, thus how would you make it an instance of Arbitrary?
The standard solution here is to create a newtype, and generate them instead. > Secondly, (and more importantly, or at least more interesting) I can see how > one would make a generator for simple compound data types, but how on earth > do you make a generator produce functions? With some difficulty, but it can be done. You could for example, select at random, a combinator or axiomatic parser, on selecting a combinator, generate a pair of parsers to hand to it with a slightly lower chance of generating a combinator and stick them together. Bob_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
