Re: [Haskell-cafe] Using multiplate to get free variables from a syntax tree

26 Feb 2012

      Here's the same code but with a variation on Multiplate that doesn't use records, but a GADT:
https://gist.github.com/1919528

It is easier on the eyes I think, but probably not any easier to decipher. But hey, this is generic programming for mutually recursive datatypes, that's a complicated subject! (Have you tried multirec?)

Sjoerd

On Feb 26, 2012, at 12:21 AM, Thomas Schilling wrote:
...
No that's correct.  I have to say the multiplate code is incredibly
hard to decipher.
On 25 February 2012 19:47, Sjoerd Visscher  wrote:
...
I don't understand what you mean.
...
...
...
($[]) . foldFor expr freeVariablesPlate $ Add (Let ("x" := Con 1) (Add (EVar "x") (EVar "y"))) (EVar "x")
(["y","x"],[])
I.e. free variables y and x, no bound variables. Is that not correct?
Sjoerd
On Feb 25, 2012, at 7:15 PM, Thomas Schilling wrote:
...
That will give you the wrong answer for an expression like:
(let x = 1 in x + y) + x
Unless you do a renaming pass first, you will end up both with a bound
"x" and a free "x".
On 25 February 2012 16:29, Sjoerd Visscher  wrote:
...
On Feb 24, 2012, at 10:09 PM, Stephen Tetley wrote:
...
I'm not familiar with Multiplate either, but presumably you can
descend into the decl - collect the bound vars, then descend into the
body expr.
...
Naturally you would need a monadic traversal
rather than an applicative one...
It turns out the traversal is still applicative. What we want to collect are the free and the declared variables, given the bound variables. ('Let' will turn the declared variables into bound variables.) So the type is [Var] -> ([Var], [Var]). Note that this is a Monoid, thanks to the instances for ((->) r), (,) and []. So we can use the code from preorderFold, but add an exception for the 'Let' case.
freeVariablesPlate :: Plate (Constant ([Var] -> ([Var], [Var])))
freeVariablesPlate = handleLet (varPlate `appendPlate` multiplate freeVariablesPlate)
 where
   varPlate = Plate {
     expr = \x -> Constant $ \bounded -> ([ v | EVar v <- [x], v `notElem` bounded], []),
     decl = \x -> Constant $ const ([], [ v | v := _ <- [x]])
   }
   handleLet plate = plate { expr = exprLet }
     where
       exprLet (Let d e) = Constant $ \bounded ->
         let
           (freeD, declD) = foldFor decl plate d bounded
           (freeE, _)     = foldFor expr plate e (declD ++ bounded)
         in
           (freeD ++ freeE, [])
       exprLet x = expr plate x
freeVars :: Expr -> [Var]
freeVars = fst . ($ []) . foldFor expr freeVariablesPlate
...
...
> freeVars $ Let ("x" := Con 42) (Add (EVar "x") (EVar "y"))
["y"]
--
Sjoerd Visscher
https://github.com/sjoerdvisscher/blog
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Sjoerd Visscher
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-- 
Push the envelope. Watch it bend.
--
Sjoerd Visscher
https://github.com/sjoerdvisscher/blog