Re: [Haskell-cafe] AST Rewriting

This is one of the problem Syntactic aims to solve, but it requires you to use a different representation of expressions (for good or bad). If you want to keep your existing representation, then you have to use a generic programming library that supports GADTs. I know at least the Spine approach supports GADTs, but the library on Hackage seems too incomplete to be useful: http://hackage.haskell.org/package/spine I don't know if there are other libraries that support GADTs. You can also have a look at CompData: http://hackage.haskell.org/package/compdata It is similar to Syntactic (i.e. requires a different representation), but it has a richer library of generic traversals. / Emil 2012-11-21 04:20, Alexander Solla skrev:

Have you read "Data types a la carte"? The 'syntactic' package implements the ideas, but it was a little dense for my purposes when I looked (I just wanted data types, a la carte; it focuses on manipulating ASTs defined a la carte). It might be what you need, or you can roll your own based on the paper.

On Tue, Nov 20, 2012 at 3:21 PM, Steve Severance mailto:sseverance@alphaheavy.com> wrote:

Hi Everyone,

I am trying to build a function to rewrite and AST. I have and AST which is designed to represent a computation graph. I will present a simplified version here designed to illustrate the problem. I have tried numerous ways of rewriting it including uniplate, recursion and Edward Kmett's implementation of plate in his lens package.

My AST is defined using GADTs as follows:

class (ReflectDescriptor a, Typeable a, Wire a) => ProtoBuf a

data Expression a b where OpenTable :: (ProtoBuf b) => Int -> Table -> Expression () b OpenFile :: (ProtoBuf b) => Int -> String -> Expression () b WriteFile :: (Typeable a, ProtoBuf b) => Int -> String -> Expression a b -> Expression b () WriteTable :: (Typeable a, ProtoBuf b) => Int -> Table -> Expression a b -> Expression b () Map :: (ProtoBuf a, ProtoBuf b, ProtoBuf c) => Int -> (a -> b) -> Expression c a -> Expression a b LocalMerge :: (ProtoBuf a) => Int -> [Expression c a] -> Expression c a

The user can create code inside a Monad Transformer like so:

q <- query $ do table <- openTable myTable transform <- map someFunc table writeTable otherTable transform

As part of this language the compiler I am building would need to for instance transform OpenTable into a series OpenFile nodes with a LocalMerge to merge the results together.

So uniplate cannot work over GADTs if I recall correctly.

I exchanged emails with Edward and he explained that for the lens case I would need something like an indexed lens family from his indexed package which is not implemented yet but which may be in the future.

The issue with recursion is that as you recurse through the AST the a b on the Expression change and GHC cannot compile it because it wants the a b to be the same on each recursive call.

My question to the Haskell community is how might one develop AST rewriting functionality. One possible solution is stripping the types away from GHC and doing all the type checking myself. That doesn't seem very good.

Another possibility that I have looked at was using hoopl. It seems very compatible given that it is built for describing and optimizing data flow which I am doing however the learning curve looks quite steep. I have been reluctant so far to invest the time in it.

Has anyone developed something similar? What recommendations do you have?

Thanks.

Steve

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