Haskell Foldable Wats (Was: Add conspicuously missing Functor instances for tuples)

Wed Feb 24 15:24:42 UTC 2016

Hi,

Sorry Nathan, I am not following your argument: you seem to be arguing two ways at the same time. Maybe "marginal" should have been "major"?

However:

> I see an additional way for an unwary
> learner to discover complexity that
> was already there, by passing a value
> that they weren't previously able to
> pass to a function

No, that specific complexity were *not* there previously. Applying length or maximum etc. to a tuple used to be a type error. End of story. And that was a good thing for many reasons, including that length or maximum on tuples are not particularly useful: the former being a roundabout way to compute 1, the latter being a rather obscurely named projection function.

/Henrik

Henrik Nilsson
School of Computer Science
The University of Nottingham
nhn at cs.nott.ac.uk

-------- Original message --------
From: Nathan Bouscal
Date:2016/02/24 14:42 (GMT+00:00)
To: Haskell Libraries
Subject: Re: Haskell Foldable Wats (Was: Add conspicuously missing Functor instances for tuples)

I don't see any additional complexity. I see an additional way for an unwary learner to discover complexity that was already there, by passing a value that they weren't previously able to pass to a function that they don't yet have a complete model of. That will definitely cause some confusion, but I'm not convinced it will cause marginal confusion, nor that such confusion is bad. This is not the only way that learners can mix up tuples and lists, and that distinction is something every learner has to understand at some point.

You can still teach tuples exactly the same way that you used to teach tuples, and you can still teach finding the length of a list exactly the same way that you used to teach it. Unless your curriculum previously included an explicit demonstration of `length (1, 2)` causing an error, I don't see why it would need to change.

On Wed, Feb 24, 2016 at 2:20 PM, Henrik Nilsson <Henrik.Nilsson at nottingham.ac.uk<mailto:Henrik.Nilsson at nottingham.ac.uk>> wrote:
I am totally with Lennart here: something that used to be conceptually very simple, clear, and, not the least, easy to teach, has become complex and muddled for little if any good reason at all. What is that if not obfuscation?

And I am not just drawing on my own experience here, but also from that of many colleagues with years and years of teaching experience.

/Henrik

Henrik Nilsson
School of Computer Science
The University of Nottingham
nhn at cs.nott.ac.uk<mailto:nhn at cs.nott.ac.uk>

-------- Original message --------
From: Nathan Bouscal
Date:2016/02/24 13:43 (GMT+00:00)
To: Haskell Libraries
Subject: Re: Haskell Foldable Wats (Was: Add conspicuously missing Functor instances for tuples)

I'm not trying to say that a pair is not a container of two things. I'm saying that that description is insufficiently specific to be useful for the purposes of the discussion. There are many ways to be a container of two things, and if we are to have functions whose behavior depends on the structure of the data they're working on, it's inevitable that those functions will behave differently for different of those ways of being a container. If the issue is that "containers" don't always behave the way one might naively expect containers to behave, then I'm just pointing out that this isn't the only place that holds, and that in other areas we've already accepted this.

On Wed, Feb 24, 2016 at 1:38 PM, Augustsson, Lennart <Lennart.Augustsson at sc.com<mailto:Lennart.Augustsson at sc.com>> wrote:
Of course a pair is a container of two things (which can have different types).
You can come up with some different definition of what it means to be a container, so that a pair is no longer a container of two things, but this is just obfuscation.

From: Libraries [mailto:libraries-bounces at haskell.org<mailto:libraries-bounces at haskell.org>] On Behalf Of Nathan Bouscal
Sent: 24 February 2016 13:29
To: Haskell Libraries
Subject: Re: Haskell Foldable Wats (Was: Add conspicuously missing Functor instances for tuples)

On Wed, Feb 24, 2016 at 1:04 PM, Henrik Nilsson <Henrik.Nilsson at nottingham.ac.uk<mailto:Henrik.Nilsson at nottingham.ac.uk>> wrote:
Hi,

On 02/24/2016 11:08 AM, Fumiaki Kinoshita wrote:
Thinking tuples of as multi-element containers is not recommended. A
tuple (a, b) is, a pair of one 'a' and one 'b';

Which, to me, at least, very much sounds like a container of two
elements?

You can use essentially the same argument to say that [a] sounds like a container of any number of elements, therefore there shouldn't be anything wrong with [1, 'foo']. It's not uncommon in programming for "what a thing naively sounds like" to be quite different from "what a thing actually is". Tuples are not lists.

I agree that there's room for confusion, but there is room for confusion in a lot of parts of Haskell, especially for people who bring a lot of preconceived notions with them. We should try to make the transition easier for them, but to me that looks a lot more like "really good error messages" and less like pointedly ignoring the structure of types that might be confusing.

Seriosuly, if, as a result of tuples being instances of Functor and
Foldable etc., the end result is confusion to the point that
many no longer understand a tuple simply as a container of a certain
number of elements, then that's another case in point against
this whole design. (In particular the Foldable part: while I personally
don't find the functor instances particularly compelling or useful,
they seem less likely to seriously bite.)
as Foldable works on
values pointed by the rightmost type argument, 1 should be the only
reasonable result of 'length'.

     data TwoThree a b = TwoThree a a b b b

What should 'length (TwoThree "Foo" "Bar" 0 1 2)' be?

A static type error, perhaps?

(As indeed it will be unless the appropriate instances are made
for TwoThree. But I am guessing we should understand TwoThree
as a tuple here.)
Looking at only
the expression, 5 might seem to make sense, but is not meaningful
considering the type.

Best,

/Henrik
--
Henrik Nilsson
School of Computer Science
The University of Nottingham
nhn at cs.nott.ac.uk<mailto:nhn at cs.nott.ac.uk>

This message and any attachment are intended solely for the addressee
and may contain confidential information. If you have received this
message in error, please send it back to me, and immediately delete it.
Please do not use, copy or disclose the information contained in this
message or in any attachment.  Any views or opinions expressed by the
author of this email do not necessarily reflect the views of the
University of Nottingham.

This message has been checked for viruses but the contents of an
attachment may still contain software viruses which could damage your
computer system, you are advised to perform your own checks. Email
communications with the University of Nottingham may be monitored as
permitted by UK legislation.

_______________________________________________
Libraries mailing list
Libraries at haskell.org<mailto:Libraries at haskell.org>
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

This email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please delete all copies and notify the sender immediately. You may wish to refer to the incorporation details of Standard Chartered PLC, Standard Chartered Bank and their subsidiaries at http://www.standardchartered.com/en/incorporation-details.html

Insofar as this communication contains any market commentary, the market commentary has been prepared by sales and/or trading desk of Standard Chartered Bank or its affiliate. It is not and does not constitute research material, independent research, recommendation or financial advice. Any market commentary is for information purpose only and shall not be relied for any other purpose, and is subject to the relevant disclaimers available at http://wholesalebanking.standardchartered.com/en/utility/Pages/d-mkt.aspx

Insofar as this e-mail contains the term sheet for a proposed transaction, by responding affirmatively to this e-mail, you agree that you have understood the terms and conditions in the attached term sheet and evaluated the merits and risks of the transaction. We may at times also request you to sign on the term sheet to acknowledge in respect of the same.

Please visit http://wholesalebanking.standardchartered.com/en/capabilities/financialmarkets/Pages/doddfrankdisclosures.aspx for important information with respect to derivative products.

This message and any attachment are intended solely for the addressee
and may contain confidential information. If you have received this
message in error, please send it back to me, and immediately delete it.

Please do not use, copy or disclose the information contained in this
message or in any attachment.  Any views or opinions expressed by the
author of this email do not necessarily reflect the views of the
University of Nottingham.

This message has been checked for viruses but the contents of an
attachment may still contain software viruses which could damage your
computer system, you are advised to perform your own checks. Email
communications with the University of Nottingham may be monitored as
permitted by UK legislation.

This message and any attachment are intended solely for the addressee
and may contain confidential information. If you have received this
message in error, please send it back to me, and immediately delete it. 

Please do not use, copy or disclose the information contained in this
message or in any attachment.  Any views or opinions expressed by the
author of this email do not necessarily reflect the views of the
University of Nottingham.

This message has been checked for viruses but the contents of an
attachment may still contain software viruses which could damage your
computer system, you are advised to perform your own checks. Email
communications with the University of Nottingham may be monitored as
permitted by UK legislation.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20160224/493f1f9a/attachment-0001.html>