[Haskell-cafe] Thoughts on program annotations.
batterseapower at hotmail.com
Fri Mar 4 08:56:55 CET 2011
On 4 March 2011 06:32, Jason Dusek <jason.dusek at gmail.com> wrote:
> -- From https://github.com/solidsnack/bash/blob/c718de36d349efc9ac073a2c7082742c45606769/hs/Language/Bash/Syntax.hs
> data Annotated t = Annotated t (Statement t)
> data Statement t = SimpleCommand Expression [Expression]
> | ...
> | IfThen (Annotated t) (Annotated t)
> | ...
I use this a variant of approach quite extensively and it works well
for me. My scheme is:
data Statement t = SimpleCommand Expression [Expression]
| IfThen (t (Statement t)) (t (Statement t))
This is a slightly more efficient representation because it lets you
unpack the "t" field of your Annotated data constructor. For example,
what would in your system would be:
type MyStatement = Statement (Int, Int)
Would in my system be:
data Ann s = Ann Int Int s
type MyStatement = Statement Ann
i.e. instead of allocating both a Statement and a (,) at each level we
allocate just a Ann at each level.
In this system you will probably find it convenient to have a
typeclass inhabited by each possible annotation type:
class Copointed t where
extract :: t a -> a
instance Copointed Ann where
extract (Ann _ _ x) = x
Anyway, this is only a minor efficiency concern -- your scheme looks
solid to me as well.
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