[Haskell-cafe] Re: QuickCheck: Arbitrary for a complex type

[Stefan O'Rear]

On Wed, Apr 04, 2007 at 10:59:45PM +0100, Joel Reymont wrote:
> One last bit then...
> 
> My identifiers should start with letter <|> char '_' and the tail  
> should be alphaNum <|> char '_'.
> 
> I guess I can use choose and oneof to produce the right set of  
> characters but how do I combine  the two into a single identifier of  
> a given length (up to 20 chars, say)?
> 
> 	Thanks, Joel
> 
> On Apr 4, 2007, at 5:27 PM, Fawzi Mohamed wrote:
> 
> >Great, just one thing that could be important : when you have  
> >recursive structures (like your Statement through Compound) be sure  
> >to use
> >sized (\mySize -> ...)
> >as generator for arbitrary so that you can avoid infinite looping.

quickcheck is a monad so you can just do:

do first <- elements $ '_' : ['a' .. 'z']
   len   <- elements $ [5..19]
   rest  <- replicateM len $ elements $ '_' : ['a' .. 'z'] ++ ['0' .. '9']
   return (first : rest)