[Haskell-cafe] Compiler's bane

[Brandon S. Allbery KF8NH]

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 at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH