I have often used the method

> allIsFine = walk $ concatMap theFine

But I don't think this will be any more efficient than your
first version with `walk (filter $ not . isThe)`.
Actually, I'd be interested in seeing benchmarks with these
approaches, if efficiency matters enough to you to research
this more.

Another possibility is to walk with a function like

killThe :: Inline -> Inline
killThe (Str "the") = Str "" -- or: Span ("",[],[]) []
killThe x = x

This should be more efficient than the list versions, though
again it would be interesting to see benchmarks.



Ilia Zaihcuk <zoi...@gmail.com> writes:

> Hi all,
>
> Using pandoc-types' Walkable
> <https://hackage.haskell.org/package/pandoc-types-1.22/docs/Text-Pandoc-Walk.html#t:Walkable>
> class, what is the best-performing/most idiomatic way to filter out certain
> elements from a document?
>
> Say I wanted to remove all occurrences of the word "the" from a Pandoc. The
> best implementation I've been able to write for this is
>
> isThe :: Inline -> Bool
> isThe (Str "the") = True
> isThe _ = False
>
> removeThe :: Pandoc -> Pandoc
> removeThe = walk (filter $ not . isThe)
>
> Is this right? Does the use of filter here not mean an additional O(n)
> traversal happens on every list of Inlines?
>
> I feel like a better solution for this would be using
>
> filterThe :: Inline -> [Inline]
> filterThe (Str "the") = []
> filterThe x = [x]
>
> or something similar, but of course
>
> removeThe = walk filterThe
>
> does not typecheck. We need a -> a, meaning [Inline] -> [Inline] here.
>
>
> The same question actually applies to any transformation which "changes the
> number of elements":
>
> theFine :: Inline -> [Inline]
> theFine (Str "the") = [Str "the", Space, Str "fine"]
> theFine i = [i]
>
> allIsFine :: Pandoc -> Pandoc
> allIsFine = walk $ concatMap theFine
>
> Best,
>
> Ilia
>
> P.S. Sorry if this has been asked before. I feel it must be a common issue,
> but all I've been able to find is this thread
> <https://groups.google.com/g/pandoc-discuss/c/idlbnOk1ooE/m/_QVvaRprHVcJ>
> with its links, which are 7 years old now and seem to be calling for
> changes in pandoc. Everything else suggests stepping outside Haskell.
>
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