I have updated the page, with a bit more detail and an additional plan
> OK. It’s hard to keep this straight in email. Take a look at
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>> https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow/Instances
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>> Please edit and improve it.
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Alan
ow/Instances
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> Please edit and improve it.
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> Simon
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> *From:* Alan & Kim Zimmerman [mailto:alan.z...@gmail.com]
> *Sent:* 13 November 2017 13:30
>
> *To:* Simon Peyton Jones <simo...@microsoft.com>
> *Subject:*
3:30
To: Simon Peyton Jones <simo...@microsoft.com>
Subject: Re: Trees that Grow and constraints
At the moment, in GHC master, we have
data HsOverLit p
= OverLit {
ol_ext :: (XOverLit p),
ol_val :: OverLitVal,
ol_witness :: HsExpr p} -- Note [Overloaded literal
Isn't the solution always
if generic programming makes things complicated, avoid it!
Here generic programming is where you define instances parametric on the
phase index.
Why not defining three instances of the type class, one per each phase?
Yes, we get code duplication (which in this case is
Just to clarify, this example is a simplification, in practice we would be
applying different type for each type instance
e.g.
type instance XEOverLit (GhcPass 'Parsed ) = PlaceHolder
type instance XEOverLit (GhcPass 'Renamed) = PlaceHolder
type instance XEOverLit (GhcPass 'Typechecked)
> So why not use one?
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> Simon
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If I do
instance (Data p) => Data (Experiment p)
then GHC does not know that the type instances for
type instance XEOverLit (GhcPass 'Parsed ) = PlaceHolder
type instance XEOverLit (GhcPass 'Renamed) = PlaceHolder
type instance
Where specifying the type instance with a wild card keeps GHC happy (the
XXOverLit case), but specifying for each of the three constructors for pass
does not (the XEOverLit case)
Well, of course! The derived data instance looks something like
instance (Data (GhcPass p)) => Data (Experiment
And it seems that
--
data Experiment p
= Experiment {
e_ext :: (XEOverLit p),
e_val :: Int }
| XExperiment (XXOverLit p)
deriving instance (Data GhcPs) => Data (Experiment GhcPs)
deriving instance (Data GhcRn) => Data (Experiment GhcRn)
deriving
