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Allow appending and then dropping the same number of elements from Stream #608

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Allow appending and then dropping the same number of elements from Stream #608

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7d0ed3c
throw `DropIndexOverflow` only when drop length exceeds app length
strikef Mar 14, 2025
9e39318
simplify `drop k (app k s)` to avoid erroneous C codegen when |drop| …
strikef Mar 14, 2025
67482fc
Move fix to `Temporal` and add explanatory comments
strikef Mar 27, 2025
b1ad228
Remove redundant diff
strikef Mar 27, 2025
c6e7a99
Fix indent and formatting
strikef Mar 27, 2025
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2 changes: 1 addition & 1 deletion copilot-language/src/Copilot/Language/Analyze.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -188,7 +188,7 @@ analyzeAppend refStreams dstn e b f = do
-- append.
analyzeDrop :: Int -> Stream a -> IO ()
analyzeDrop k (Append xs _ _)
| k >= length xs = throw DropIndexOverflow
| k > length xs = throw DropIndexOverflow
| k > fromIntegral (maxBound :: DropIdx) = throw DropMaxViolation
| otherwise = return ()
analyzeDrop _ _ = throw DropAppliedToNonAppend
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9 changes: 6 additions & 3 deletions copilot-language/src/Copilot/Language/Reify.hs
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Expand Up @@ -147,9 +147,12 @@ mkExpr refMkId refStreams refMap = go
------------------------------------------------------

Drop k e1 -> case e1 of
Append _ _ _ -> do
s <- mkStream refMkId refStreams refMap e1
return $ Core.Drop typeOf (fromIntegral k) s
Append a _ s ->
if k == length a
then go s
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Can you include a comment here saying why we need a special case for k == length a? It wasn't obvious to me until I read the PR description.

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Relatedly, I wonder if Copilot.Language.Reify if the right place to implement this transformation. Most of the simplifications that are performed over Drop streams are implemented in the drop function itself:

copilot/copilot-language/src/Copilot/Language/Operators/Temporal.hs

Lines 35 to 39 in e042933

drop :: Typed a => Int -> Stream a -> Stream a
drop 0 s = s
drop _ ( Const j ) = Const j
drop i ( Drop j s ) = Drop (fromIntegral i + j) s
drop i s = Drop (fromIntegral i) s

Perhaps we should implement this transformation as an additional case of drop?

drop i ( Append a _ s ) | i == length a = s

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@ivanperez-keera ivanperez-keera Mar 26, 2025 •
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Not part of this issue, but that might be a good place to also do something like:

drop i (Op1 op s)     = Op1 op (drop i s)
drop i (Op2 op s1 s2) = Op2 op (drop i s1) (drop i s2)

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@strikef strikef Mar 27, 2025 •
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I agree that Reify module isn't the best place to implement this fix. The Temporal module you recommended looks better to me as well.

My initial intent of this change was getting correct C code (which is why I placed this fix at Reify module), and at that time simplification was just a 'nice' side-effect. But now that you mentioned, I don't see any reason to not treat it as simplification.
I'll move my fix to Temporal module and add explanatory comment as well.

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Fixed as suggested

else do
_s <- mkStream refMkId refStreams refMap e1
return $ Core.Drop typeOf (fromIntegral k) _s
_ -> impossible "mkExpr" "copilot-language"

------------------------------------------------------
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