Haskell Foldable Wats

Andreas Abel abela at chalmers.se
Wed Feb 24 08:57:41 UTC 2016

In my n+1 years of participating in a 100kloc Haskell project, I learned 
that programming with the "categorical" data type building blocks 
"Either" and tuples leads to code that is hard to read and maintain.  It 
is tempting to use a pair

   type QNamed a = (QName, a)

instead of a hand-rolled data type

   data QNamed a = QNamed { qname :: QName, qnamedThing :: a }

since with pairs, I get lots of operations for free, and even more if I 
use Functor and Traversable, it seems.  However, in the long run, if I 
come back after months to my original code, or even worse, to someone 
else's code, I dearly pay for this:

   1. Harder to read the code, as I may only see the non-telling "fst" 
and "snd" instead of the semantics-loaden "qname" and "qnamedThing".

   2. Lack of code exploration facilities like grep and tags.  I can 
grep for "QName" and "qname", but grepping for "," and "fst" returns 
lots of irrelevant locations.

   3. Non-telling error messages.

And all the arguments of using newtypes over type synonyms apply here as 

So, my advice is to use tuples only for very short-lived data, like 
returning multiple results from a function.  I am speaking here of 
Haskell as a language for software development, not of Haskell as a 
realization of category theory.

For getting a broader acceptance of Haskell as a language for software 
development, Foldable on tuples is not helpful.

And yes, if I was new to Haskell and learned the GHCI thinks that

   minimum (1,2) == 1

then I would advice GHCI to visit some mental institution to get its 
delusions fixed.


On 23.02.2016 18:29, Nathan Bouscal wrote:
> Sorry, poor quoting on my part. I was attempting to reply to Andreas's
> earlier points:
>  >> Use of tuples is highly discourageable…
>>>I see no point in Functor or Foldable for tuples.
> On Tue, Feb 23, 2016 at 5:18 PM, Mario Blažević <mblazevic at stilo.com
> <mailto:mblazevic at stilo.com>> wrote:
>     On 16-02-23 11:23 AM, Nathan Bouscal wrote:
>         It's a bit bold to simultaneously say "Nobody should use this
>         type," and
>         also "If you use this type, you should do it this way." If you think
>         that it's bad practice to use tuples, that's a fine and respectable
>         opinion, but at that point you should start being a bit more
>         wary about
>         telling those who think otherwise /how/ they should use them.
>              I believe he was referring to lists, not to tuples. There
>     were  few Prelude functions limited to tuples before FTP, and those
>     haven't changed.
>         On Tue, Feb 23, 2016 at 1:10 PM, Marcin Mrotek
>         <marcin.jan.mrotek at gmail.com
>         <mailto:marcin.jan.mrotek at gmail.com>
>         <mailto:marcin.jan.mrotek at gmail.com
>         <mailto:marcin.jan.mrotek at gmail.com>>> wrote:
>              > I can assure you that Andreas is not delusional
>              I didn't say that. I was referring to his response to my
>         post, that
>              "Some delusions have very sophisticated explanations."
>              > You have to show good taste or you'll end up with a mess
>         that might be logically consistent, but unpleasant to use.
>              This is entirely subjective, and frankly I don't think that
>         post-FTP
>              Haskell is a "mess" or is "unpleasant to use". If anything,
>         it became
>              more useful to me, because now Prelude functions aren't
>         limited to the
>              one data structure I almost never use.
>              Best regards,
>              Marcin Mrotek
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Andreas Abel  <><      Du bist der geliebte Mensch.

Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden

andreas.abel at gu.se

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