No that's correct. I have to say the multiplate code is incredibly
hard to decipher.
On 25 February 2012 19:47, Sjoerd Visscher <sjo...@w3future.com> 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 <sjo...@w3future.com> 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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>>> Haskell-Cafe mailing list
>>> Haskell-Cafe@haskell.org
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>>
>>
>>
>> --
>> Push the envelope. Watch it bend.
>>
>
> --
> Sjoerd Visscher
> https://github.com/sjoerdvisscher/blog
>
>
>
>
>
>
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> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org
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--
Push the envelope. Watch it bend.
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