[Haskell] Re: [Haskell-cafe] ANN: cmonad 0.1.1

Max Bolingbroke batterseapower at hotmail.com
Mon Mar 30 19:20:27 EDT 2009

2009/3/30 Don Stewart <dons at galois.com>:
> Duncan and I have thought about this too, exactly as you describe.
>    (Just !x)
>  =>
>    (# tag#, x# #)

It would be nice to generalize this to arbitrary sum types, but doing
so plays hell with the type checker - I think the most straightforward
way would be to extend Core with a type-level case statement on tag
values similar to \Pi\Sigma.

BTW you can manually apply the worker/wrapper transform to arrayU to
realise this optimisation in todays GHC. However, I couldn't get GHC
to inline the definitions of (-) etc from Ops though - seems to be an
issue with it throwing away INLINE annotations on the appropriate
dictionary members of Num.

If both of these things were fixed you would /still/ get bad code,
because mapM (used in arrayU) does not produce a fusible list, so the
resulting program still builds the lists that masquerade as array
indexes and then folds over them. Disappointing, but this can also be
solved, as long as you apply the monadic stream technology from
Roman's vector package.

Apologies for the brevity of these notes, but perhaps they make some
of the issues clearer. GHC really should be doing much better here :-(


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