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
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> *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
lan & Kim
Zimmerman
Sent: 13 November 2017 09:24
To: ghc-devs@haskell.org
Subject: Trees that Grow and constraints
At the moment the Trees that Grow implementation in GHC master makes use of
massive constraint types to provide Data instances for the hsSyn AST.
I am trying to remove t
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
At the moment the Trees that Grow implementation in GHC master makes use of
massive constraint types to provide Data instances for the hsSyn AST.
I am trying to remove the need for this, and have hit a problem.
The example I have reduced it to is here [1]
The essence of the problem is
