Haskell Foldable Wats

Augustsson, Lennart Lennart.Augustsson at sc.com
Wed Feb 24 16:08:21 UTC 2016

Having length of a tuple not be a type error is not only confusing for beginners.
It can also mask bugs in real code written by experts.  And it’s already happened.
Since length of a tuple returning 1 is nonsensical and useless, it should simply not be legal.

From: Libraries [mailto:libraries-bounces at haskell.org] On Behalf Of Nathan Bouscal
Sent: 24 February 2016 15:56
To: Kosyrev Serge
Cc: Haskell Libraries
Subject: Re: Haskell Foldable Wats

On Wed, Feb 24, 2016 at 3:49 PM, Kosyrev Serge <_deepfire at feelingofgreen.ru<mailto:_deepfire at feelingofgreen.ru>> wrote:
Nathan Bouscal <nbouscal at gmail.com<mailto:nbouscal at gmail.com>> writes:
> Apologies for being unclear. By marginal, I meant that I expect many
> learners who end up confused by this would have otherwise come across
> the same confusion by other paths.

This sounds odd to me in two ways:

1. it is not "the same" confusion -- it is a worse confusion, for
   several reasons:
   - the questionable Functor instance gives a cognitive dissonance for
     not transforming the entirety of the tuple (for good reasons, but still!)
   - the questionable Functor is a puzzlingly arbitrary choice among a
     N-family of options that an N-tuple provides.

As Ed pointed out, there is nothing arbitrary about the choice. This actually makes tuples in some sense a uniquely good way of explaining how things work, because you can show why this restriction exists.

2. but even if the confusion itself wasn't made worse -- its impact has
   increased, because exposure to it has increased

To the extent that exposure has increased, I would think it would be caused by confusion of the "lists vs tuples" type, which seems important to resolve early. That resolution doesn't really need to say anything about how Functor works, and can instead be more along the lines of "What problem were you trying to solve by taking the length of a tuple? Should you have been using a list instead?"

с уважениeм / respectfully,
Косырев Сергей

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