Debug Haskell composition chain without converting to point-full representation

Question

I have learned that point-free style is preferred in the Haskell community, and I often write expressions like this:

naive = (slugifyUnicode . T.take maxFilenameSize . T.pack . stripHtmlTags . T.unpack . plainToHtml) sfld

However, while debugging, I find myself repeatedly converting expressions like this into chains of $ operators in order to use some variant of trace, and I am starting to think it is preferable to forgo the point-free style and just write lots of $s to start with, in order to make debugging less cumbersome. After all, it ends up being about the same number of characters.

Does anyone know of a way to debug long chains of composed functions without de-composing them?

And more generally, any comments on this inconvenience are very welcome.

"I have learned that point-free style is preferred in the Haskell community" -- note that's not always the case. From what I observed, the community loves point-free style only when it leads to elegant and clear code. However, in many cases applying point-free leads to a mess, and it is jokingly referred to as "point-less style" by the community. In your case, the code is nice, but be careful. Also, f . g . h $ x works as expected and is quite idiomatic. — chi
– chi, Commented Dec 26, 2022 at 11:43
What would naive look like after you un-point-free'd it and inserted a trace? — Daniel Wagner
– Daniel Wagner, Commented Dec 26, 2022 at 14:40
@LouisWasserman I would be happy to be told I'm wrong. I am not an oracle for the consensus opinion of all users, obviously. — Brendan Langfield
– Brendan Langfield, Commented Dec 26, 2022 at 15:07
@DanielWagner I would usually break half of it into an intermediate value defined in a where clause, trace it there, and then do the rest of the computation at the original site of the expression. — Brendan Langfield
– Brendan Langfield, Commented Dec 26, 2022 at 15:08

cafce25 · Accepted Answer · 2022-12-26 06:25:11Z

3

For any intermediate value that has a Show instance you can just use traceShowId inline:

naive = (slugifyUnicode . T.take maxFilenameSize . traceShowId . T.pack . stripHtmlTags . T.unpack .plainToHtml) sfld

If the intermediary value is a String you can use traceId instead.

For anything that doesn't have a Show instance you'd have to define a helper:

data CustomType = CustomType String
traceHelper :: CustomType -> CustomType
traceHelper s@(CustomType c) = trace c $ s

-- arbitrary functions we want to compose
a :: aIn -> CustomType
b :: CustomType -> bOut

c :: aIn -> bOut
c = b . traceHelper . a

answered Dec 26, 2022 at 6:25

cafce25

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