[Haskell-cafe] Compiler's bane

[Andrew Coppin]

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.