On Sep 4, 2008, at 13:41 , Andrew Coppin wrote:
Ryan Ingram wrote:
It's pretty simple, I think.
type ExpGen = ReaderT [String] Gen
arbExp :: ExpGen Expression -- exercise for the reader
instance Arbitrary Expression where arbitrary = runReaderT arbExp [] coarbitrary = coarbExp
coarbExp (Var s) = variant 0 . coarbitrary s coarbExp (Apply a b) = variant 1 . coarbitrary a . coarbitrary b coarbExp (Lambda s e) = variant 2 . coarbitrary s . coarbitrary e
instance Arbitrary Char where arbitrary = elements "abcdefghijklmnopqrstuvwxyz_" coarbitrary = coarbitrary . fromEnum
o_O
I love the way other people have wildly different ideas of "simple" than me. I'm staring at this and completely failing to comprehend it. (But then, anything with "co" in the name generally makes little sense to me...) Why on earth would you need a reader monad? Surely if you want to add bound variables and then later query what variables are bound, you'd want a state monad? Hmm, I'm completely lost here.
Reader, in this case, is a State monad with the addition of scopes: to create a new nested scope r' given a scope r, "let r' = local r". The [String] is a list of variable names, if this is doing what I think it is. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH