Add FreeTrace.hs #10
Add FreeTrace.hs #10
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| → EitherT (ErrorTrace t e) m α | ||
| runTraceT = run S.empty . _traceT | ||
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| run ∷ Monad m ⇒ Seq t → ProgramT (TraceF t e m) m α → EitherT (ErrorTrace t e) m α |
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Can we use the multi-line style for run (like runTraceT)?
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Sure. For go as well or just run since it's top-level?
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Hmmm, to avoid it looking to gnarly, I'd probably leave go as is; I don't really have a set practice for stuff in where-clauses. Also, would you indent go by two spaces below the where-clause? (Sorry for all the nitpicking!)
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This is very elegant! |
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Thanks. Does it seem to work as expected or do you need those instances before trying it? |
| {-# LANGUAGE UnicodeSyntax #-} | ||
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| module Control.Monad.Trans.FreeTrace | ||
| where |
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Just a reminder to fix the exports.
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When I get some time today, I'll try and write a little demo with this version to make sure it does what I expect; it looks right, fwiw. |
| Scope ∷ t → TraceT t e m α → TraceF t e m α | ||
| Throw ∷ e → TraceF t e m α | ||
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| -- | The Free Trace Monad. |
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Call it the Free Trace monad transformer and drop the punctuation.
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Do you have any thoughts on perhaps a less general name than Trace here? Since I would expect the "Free Trace monad (transformer)" to be something like the smallest possible implementation of MonadTrace, but this also has error-handling semantics. I couldn't think of anything, so I called my thing TraceT, but I think it's not optimal.
If you can't think of anything, I'm fine deferring the name-change to a different time.
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Hmm, good question. I don't have a suggestion just now. One possibility would be to define free Trace without error-handling along with another free monad just for error handling and then composing their interpreters. This is pretty easy to do with free monads I think (although I haven't tried specifically with operational). That way you'd have "the smallest trace thing" along with "the smallest trace thing with error-handling".
It's also worth mentioning that there is already something called Trace but it's a comonad. I think it's CoWriter. Maybe I should figure out how it works…
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I should mention that the only reason I put the error-handling stuff in there was to be able to define the MonadError instance with a single structure. But if you're okay with dropping that requirement, having a more minimal structure (or two) seems like a good idea.
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I think there's a bug in the current trace semantics on |
No problem. |
| -- showParen (p > 10) $ | ||
| -- foldr (.) id (intersperse (" ∥ "++) $ (foldr (.) id . intersperse ('.':) . fmap shows . F.toList) <$> _etTrace) | ||
| -- . (" ⇑ " ++) | ||
| -- . shows _etError |
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Might want to add a Foldable mt constraint to restore some sort of structured printing like this.
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I'm going to go ahead and close the PR but feel free to ping me if you find a way this can work out. |
ghost
closed this
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OK. I think that the fact that it is so non-trivial to implement |
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No worries. Indeed, I think the If you're looking for some yaks to shave, Conor has an interesting draft: Why walk when you can take the tube?. Capturing state as a kind of zipper for |
Following up on the discussion in #9. It compiles but I haven't tested it.