From: John MacFarlane <jgm-TVLZxgkOlNX2fBVCVOL8/A@public.gmane.org>
To: Ilia Zaihcuk <zoickx-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
pandoc-discuss
<pandoc-discuss-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org>
Subject: Re: Removing parts of the document with [walk] in Haskell
Date: Sat, 04 Sep 2021 22:23:03 -0700 [thread overview]
Message-ID: <m2mtorlhoo.fsf@MacBook-Pro-2.hsd1.ca.comcast.net> (raw)
In-Reply-To: <915213fc-e4c4-480b-a6a2-fd3420777ddan-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org>
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 <zoickx-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> 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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next prev parent reply other threads:[~2021-09-05 5:23 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-04 19:09 Ilia Zaihcuk
[not found] ` <915213fc-e4c4-480b-a6a2-fd3420777ddan-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org>
2021-09-04 19:26 ` Gwern Branwen
2021-09-05 5:23 ` John MacFarlane [this message]
[not found] ` <m2mtorlhoo.fsf-jF64zX8BO0+FqBokazbCQ6OPv3vYUT2dxr7GGTnW70NeoWH0uzbU5w@public.gmane.org>
2021-09-05 15:09 ` Ilia Zaihcuk
[not found] ` <1473b62b-4b33-49b5-ac6d-52a5571d8068n-/JYPxA39Uh5TLH3MbocFFw@public.gmane.org>
2021-09-05 15:42 ` Gwern Branwen
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