Removing parts of the document with [walk] in Haskell

[Ilia Zaihcuk <zoickx-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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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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