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View Code? Open in Web Editor NEWOpen algebraic data types. Reuse constructors in different types; add and remove constructors from values.
License: BSD 3-Clause "New" or "Revised" License
Open algebraic data types. Reuse constructors in different types; add and remove constructors from values.
License: BSD 3-Clause "New" or "Revised" License
The tutorial says:
These situations can be handled with the type class approach just discussed with overlappable instances.
Could you give an example for those of us not expert in type hackery ? I have some difficulty writing the Overlappable instance. A draft would be something like:
instance {-# OVERLAPPABLE #-} ( OpenAlg r ?? ?? (OpenADT r)) => OverList a a' r ?? where
fmapList' _ ?? = ??
Thanks !
Assuming that all xs are Ord.
Here is a sample code:
replaceVariable :: ( OpenAlg xs "atomF" AtomF (OpenADT xs)
, (ApplyRow (OpenADT xs) xs .- "atomF") ~ ApplyRow (OpenADT xs) (xs .- "atomF")
, ((xs .- "atomF") .\/ 'R '["atomF" ':-> AtomF]) ~ xs
, (ApplyRow (OpenADT xs) (xs .- "atomF") .\/ 'R '["atomF" ':-> AtomF (Fix (VarF xs))])
~ ApplyRow (OpenADT xs) xs
, (Forall (xs .- "atomF") Functor)
)
=> Text -> Term -> OpenADT xs -> OpenADT xs
replaceVariable variable value formula =
case trialF (unfix formula) #atomF of
Right other -> other & (fmap (replaceVariable variable value)) & diversifyF @("atomF" .== AtomF) & Fix
Left (AtomF' α name terms) -> Atom α name $ fmap go terms
where go t =
case t of
GText text -> if text == variable then value else t
_ -> t
Some of the constraints in the signature seem to be stating the obvious.
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