Hi,
there's quite some material on how to debug/track rewrite rules that
do fire. However, I couldn't find information about how to find out
what is wrong with a rule that supposedly should, but actually doesn't
fire. Concretely, I would like to add a rewrite rule that converts
mconcat . map (fromWrite w)
to
fromWriteList w
The relevant definitions and types are:
mconcat = foldr append empty
append :: Builder -> Builder -> Builder
empty :: Builder
fromWrite :: Write a -> a -> Builder
fromWriteList :: Write a -> [a] -> Builder
The rewrite rule that I tried is
"foldr/fromWrite" forall w.
foldr (\x b -> append (fromWrite w x) b) empty = fromWriteList w
combined with {-# INLINE [1] #-} anotations on the `append`, `empty`,
`fromWrite` and `fromWriteList`. However, the rule doesn't fire for a
definition like
word8s :: [Word8] -> Builder
word8s = mconcat . map (fromWrite writeWord8)
I suspect that the foldr/build fusion rules are interacting. However,
I don't know how this interaction looks like. Is there a way to track
all simplifications of `word8s` in all phases? If there was, then I
could formulate my rule such that it is reduced with respect to the
other rewriting rules. Then, the interaction should work out.
thanks for your help,
Simon
PS: The above definitions are part of the builder for the bytestring
package [1] and my experiment on rule firings can be found here [2].
[1] https://github.com/meiersi/bytestring
[2]
https://github.com/meiersi/bytestring/blob/master/tests/builder/WriteListFusion.hs
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