Mark partial functions as such

[Richard Eisenberg]

Proposal: Mark partial functions in `base` as partial

Motivation: I'm about to teach Haskell to a classful of beginners. In my experience, they will soon reach for functions like `head` and `tail`, because pattern-matching is foreign to them. I would love just to be able to say "Don't use partial functions", but many students will not easily be able to tell partial functions from total ones.

I do expect this problem to work itself out rather quickly, and then students will be able to identify partial functions, but loudly marking partial functions as partial seems like a small service to everyone and a bigger one to newbies. I don't see any downsides.

Thoughts?

Thanks,
Richard

[Daniel Cartwright]

+1, I've always thought it should be like this

Post by Richard Eisenberg
Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my
experience, they will soon reach for functions like `head` and `tail`,
because pattern-matching is foreign to them. I would love just to be able
to say "Don't use partial functions", but many students will not easily be
able to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then
students will be able to identify partial functions, but loudly marking
partial functions as partial seems like a small service to everyone and a
bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
_______________________________________________
Libraries mailing list
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

[Richard Eisenberg]

Just to clarify here: all I mean is that we should include the word "Partial" in the Haddock documentation -- no deprecation or warning, just documentation.

Richard

Post by Richard Eisenberg
Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my experience, they will soon reach for functions like `head` and `tail`, because pattern-matching is foreign to them. I would love just to be able to say "Don't use partial functions", but many students will not easily be able to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then students will be able to identify partial functions, but loudly marking partial functions as partial seems like a small service to everyone and a bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
_______________________________________________
Libraries mailing list
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

[Daniel Cartwright]

That's fine, still a +1 from me

Post by Richard Eisenberg
Just to clarify here: all I mean is that we should include the word
"Partial" in the Haddock documentation -- no deprecation or warning, just
documentation.
Richard

Post by Richard Eisenberg
Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my

experience, they will soon reach for functions like `head` and `tail`,
because pattern-matching is foreign to them. I would love just to be able
to say "Don't use partial functions", but many students will not easily be
able to tell partial functions from total ones.

Post by Richard Eisenberg
I do expect this problem to work itself out rather quickly, and then

students will be able to identify partial functions, but loudly marking
partial functions as partial seems like a small service to everyone and a
bigger one to newbies. I don't see any downsides.

Post by Richard Eisenberg
Thoughts?
Thanks,
Richard
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[Daniel Díaz Casanueva]

+1 from me too. The partiality of a function seems to me like something
that should be documented.

Best,
Daniel

Am Fr., 31. Aug. 2018 um 02:10 Uhr schrieb Richard Eisenberg <

Post by Richard Eisenberg
Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my
experience, they will soon reach for functions like `head` and `tail`,
because pattern-matching is foreign to them. I would love just to be able
to say "Don't use partial functions", but many students will not easily be
able to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then
students will be able to identify partial functions, but loudly marking
partial functions as partial seems like a small service to everyone and a
bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
_______________________________________________
Libraries mailing list
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

[David Feuer]

Yes, I think so. What about functions like length? length (repeat ()) is
bottom. repeat () is not bottom. Ergo, length is partial. But I don't think
we want to say that!

Post by Daniel Díaz Casanueva
+1 from me too. The partiality of a function seems to me like something
that should be documented.
Best,
Daniel
Am Fr., 31. Aug. 2018 um 02:10 Uhr schrieb Richard Eisenberg <

Post by Richard Eisenberg
Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my
experience, they will soon reach for functions like `head` and `tail`,
because pattern-matching is foreign to them. I would love just to be able
to say "Don't use partial functions", but many students will not easily be
able to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then
students will be able to identify partial functions, but loudly marking
partial functions as partial seems like a small service to everyone and a
bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
_______________________________________________
Libraries mailing list
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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Libraries mailing list
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

[Daniel Díaz Casanueva]

Why not? I don't think mentioning that length doesn't work with infinite
lists will do any harm.

I think many people make a distinction between partiality due to endless
evaluation and partiality due to a call to "error". But I still think
documenting either of both things can be helpful.

Best,
Daniel

Am Fr., 31. Aug. 2018 um 04:09 Uhr schrieb David Feuer <

Post by David Feuer
Yes, I think so. What about functions like length? length (repeat ()) is
bottom. repeat () is not bottom. Ergo, length is partial. But I don't think
we want to say that!
On Thu, Aug 30, 2018, 10:05 PM Daniel Díaz Casanueva <

Post by Daniel Díaz Casanueva
+1 from me too. The partiality of a function seems to me like something
that should be documented.
Best,
Daniel
Am Fr., 31. Aug. 2018 um 02:10 Uhr schrieb Richard Eisenberg <

Post by Richard Eisenberg
Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my
experience, they will soon reach for functions like `head` and `tail`,
because pattern-matching is foreign to them. I would love just to be able
to say "Don't use partial functions", but many students will not easily be
able to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then
students will be able to identify partial functions, but loudly marking
partial functions as partial seems like a small service to everyone and a
bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
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[Eric Mertens]

I think this comes down to just documenting things like how strict functions are and how they behave on various classes of inputs. These are good things to document. It doesn’t just have to be about a boolean flag “partial” paste on a bunch of definitions.

Why not? I don't think mentioning that length doesn't work with infinite lists will do any harm.
I think many people make a distinction between partiality due to endless evaluation and partiality due to a call to "error". But I still think documenting either of both things can be helpful.
Best,
Daniel
Yes, I think so. What about functions like length? length (repeat ()) is bottom. repeat () is not bottom. Ergo, length is partial. But I don't think we want to say that!
+1 from me too. The partiality of a function seems to me like something that should be documented.
Best,
Daniel
Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my experience, they will soon reach for functions like `head` and `tail`, because pattern-matching is foreign to them. I would love just to be able to say "Don't use partial functions", but many students will not easily be able to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then students will be able to identify partial functions, but loudly marking partial functions as partial seems like a small service to everyone and a bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
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[David Feuer]

Yup! Just wanted to make sure it didn't end up being the latter.

Post by Eric Mertens
I think this comes down to just documenting things like how strict
functions are and how they behave on various classes of inputs. These are
good things to document. It doesn’t just have to be about a boolean flag
“partial” paste on a bunch of definitions.
Why not? I don't think mentioning that length doesn't work with infinite
lists will do any harm.
I think many people make a distinction between partiality due to endless
evaluation and partiality due to a call to "error". But I still think
documenting either of both things can be helpful.
Best,
Daniel
Am Fr., 31. Aug. 2018 um 04:09 Uhr schrieb David Feuer <

Post by David Feuer
Yes, I think so. What about functions like length? length (repeat ()) is
bottom. repeat () is not bottom. Ergo, length is partial. But I don't think
we want to say that!
On Thu, Aug 30, 2018, 10:05 PM Daniel Díaz Casanueva <

Post by Daniel Díaz Casanueva
+1 from me too. The partiality of a function seems to me like something
that should be documented.
Best,
Daniel
Am Fr., 31. Aug. 2018 um 02:10 Uhr schrieb Richard Eisenberg <

Post by Richard Eisenberg
Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In
my experience, they will soon reach for functions like `head` and `tail`,
because pattern-matching is foreign to them. I would love just to be able
to say "Don't use partial functions", but many students will not easily be
able to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then
students will be able to identify partial functions, but loudly marking
partial functions as partial seems like a small service to everyone and a
bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
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[Frerich Raabe]

Hi David,

What about functions like length? length (repeat ()) is bottom. repeat () is
not bottom. Ergo, length is partial.

This caught me by surprise - I would have never considered 'length' to be a
partial function! Maybe I don't quite understand what it means for some
expression to be 'bottom' (I thought that's the same as 'undefined').

My naive understanding was that a partial function is one which has no
definition for certain arguments; in particular, it has no definition which
could be used while doing equational reasoning by hand, on a piece of paper
(i.e. without running the program).

It appears that this is not quite correct -- instead, any function which
fails to return anything (at runtime!) for certain arguments is partial? E.g.
'sort' would be partial or even 'elem' (consider 'True `elem` repeat False')?

[Henrik Nilsson]

Hi,

Post by Frerich Raabe

What about functions like length? length (repeat ()) is bottom.
repeat () is not bottom. Ergo, length is partial.

This caught me by surprise - I would have never considered 'length'
to be a partial function! Maybe I don't quite understand what it
means for some expression to be 'bottom' (I thought that's the same
as 'undefined').

We have to be a bit careful with attributing blame for failure.
By David's argument, e.g. "fst" would also be "partial". Consider:

(_|_, _|_) /= _|_

But

fst (_|_, _|_) == _|_

But the bottom here does not originate in the computation of "fst" as
such, but in the computation of the *argument* to "fst".

Similarly for "length" above: "length" is not to blame for the
fact that

length (repeat ()) == _|_

It's simply that computation of the argument to length takes a very(!)
long time.

In a language with strict semantics, this is of course no surprise
at all, and I suspect no one would suggest that a function
like "length" is partial in a strict setting just because the
overall computation fails to terminate when the computation of
an argument does.

Post by Frerich Raabe
My naive understanding was that a partial function is one which has
no definition for certain arguments; in particular, it has no
definition which could be used while doing equational reasoning by
hand, on a piece of paper (i.e. without running the program).
It appears that this is not quite correct -- instead, any function
which fails to return anything (at runtime!) for certain arguments is
partial? E.g. 'sort' would be partial or even 'elem' (consider 'True
`elem` repeat False')?

I'd say neither "sort" nor "elem" is partial for the same reason.

As to the original suggestion of marking functions as partial, I think
that's fine, as long as one is careful to explain exactly what is meant.
(And documenting (different forms of) strictness would be fine too.)

But one should bear in mind that there are functions that are
partial for other reasons that pattern matching failure, in particular
numerical functions like division, square root, ...
(And the full story of floating point arithmetic with infinities
and NaNs etc. is of course quite complicated.)

Students (well, any programmer) should of course be aware that one have
to be extra careful when using partial functions, and certainly also
encouraged to seek alternative formulations, but just saying "Don't use
partial functions" is not the full story.

Best,

/Henrik





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[Wolfgang Jeltsch]

Post by Frerich Raabe
This caught me by surprise - I would have never considered 'length' to
be a partial function! Maybe I don't quite understand what it means
for some expression to be 'bottom' (I thought that's the same as
'undefined').
My naive understanding was that a partial function is one which has no
definition for certain arguments; in particular, it has no definition
which could be used while doing equational reasoning by hand, on a
piece of paper (i.e. without running the program).
It appears that this is not quite correct -- instead, any function
which fails to return anything (at runtime!) for certain arguments is
partial? E.g. 'sort' would be partial or even 'elem' (consider 'True
`elem` repeat False')?

The word “partial” might not have a precise definition in the context of
Haskell. In particular, it might not necessarily be defined in terms of
⊥ (bottom). However, the notion of ⊥ itself does have a precise
definition.

⊥ is a special value that every type contains. A consequence of this is
that there are also values like ⊥ : ⊥.

A good way to think about ⊥ is that ⊥ marks the absence of any
information. So the value of an expression is ⊥ if there is a lack of an
appropriate alternative in a case distinction but also if there is a
recursion that doesn’t produce any data.

For example, if zeros is defined via the equation zeroes = 0 : zeroes,
you know that zeroes must be of the form 0 : _; so it cannot be ⊥.
However, if unknown is defined via the equation unknown = unknown, there
is nothing you can learn about any information that unknown would
contain; so unknown is ⊥.

Mathematically, the values of each type form a domain such that ⊥ is the
minimum and each data constructor is an order-preserving function. When
defining a value recursively, Haskell will give you the *least* solution
of the defining equation. The equation zeros = 0 : zeros has only one
solution (0 : 0 : 0 : …). The equation unknown = unknown, on the other
hand, has every value as a solution, and thus the least of them, ⊥, is
picked as the value for unknown.

All the best,
Wolfgang

[David Feuer]

I think it actually can be made precise, with some more knowledge than I
have. Roughly speaking, a function is total if its result is fully defined
(contains no bottoms) whenever its argument is fully defined.

Post by Frerich Raabe
This caught me by surprise - I would have never considered 'length' to
be a partial function! Maybe I don't quite understand what it means
for some expression to be 'bottom' (I thought that's the same as
'undefined').
My naive understanding was that a partial function is one which has no
definition for certain arguments; in particular, it has no definition
which could be used while doing equational reasoning by hand, on a
piece of paper (i.e. without running the program).
It appears that this is not quite correct -- instead, any function
which fails to return anything (at runtime!) for certain arguments is
partial? E.g. 'sort' would be partial or even 'elem' (consider 'True
`elem` repeat False')?

The word “partial” might not have a precise definition in the context of
Haskell. In particular, it might not necessarily be defined in terms of
⊥ (bottom). However, the notion of ⊥ itself does have a precise
definition.
⊥ is a special value that every type contains. A consequence of this is
that there are also values like ⊥ : ⊥.
A good way to think about ⊥ is that ⊥ marks the absence of any
information. So the value of an expression is ⊥ if there is a lack of an
appropriate alternative in a case distinction but also if there is a
recursion that doesn’t produce any data.
For example, if zeros is defined via the equation zeroes = 0 : zeroes,
you know that zeroes must be of the form 0 : _; so it cannot be ⊥.
However, if unknown is defined via the equation unknown = unknown, there
is nothing you can learn about any information that unknown would
contain; so unknown is ⊥.
Mathematically, the values of each type form a domain such that ⊥ is the
minimum and each data constructor is an order-preserving function. When
defining a value recursively, Haskell will give you the *least* solution
of the defining equation. The equation zeros = 0 : zeros has only one
solution (0 : 0 : 0 : 
). The equation unknown = unknown, on the other
hand, has every value as a solution, and thus the least of them, ⊥, is
picked as the value for unknown.
All the best,
Wolfgang
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[Andrew Butterfield]

Dear David,

given that "data [a] = [] | (a : [a])" in Haskell is viewed co-inductively and hence admits infinite lists,
then any function f : [a] -> b is total only if it returns a result.
Does this mean it must terminate?

In a strict world, yes.

In the lazy world, it's a little more complicated than that.

Consider map id :: [a] -> [a]

is map id partial? It won't terminate if given an infinite list, but it will produce partial results on demand indefinitely - so I say it is total.

However , length applied to [0..] (say) will never return any partial or complete result, and so I would say it's partial.

I too am going to start teaching Haskell newbies, so this is of interest - to what extend to we use "stories for children"

One suggestion: if you don't start with laziness, and they initially consider lists as finite, then length :: [a] -> Int is total, where [a] is interpreted as finite lists.
When laziness enters the picture, then points out that having [a] include infinite lists means that some hitherto total function become partial, on that expanded domain.

Perhaps the added documentation should also comment for list and ADT based functions where the infinite forms influence totality/partiality?

Regards, Andrew

Yes, I think so. What about functions like length? length (repeat ()) is bottom. repeat () is not bottom. Ergo, length is partial. But I don't think we want to say that!
+1 from me too. The partiality of a function seems to me like something that should be documented.
Best,
Daniel
Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my experience, they will soon reach for functions like `head` and `tail`, because pattern-matching is foreign to them. I would love just to be able to say "Don't use partial functions", but many students will not easily be able to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then students will be able to identify partial functions, but loudly marking partial functions as partial seems like a small service to everyone and a bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
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Andrew Butterfield Tel: [phone] Fax: [phone]
***@TCD, Head of Foundations & Methods Research Group
School of Computer Science and Statistics,
Room G.39, O'Reilly Institute, Trinity College, University of Dublin
http://www.scss.tcd.ie/Andrew.Butterfield/
--------------------------------------------------------------------

[Andrew Butterfield]

+1 from me too.

How about also adding in (to the documentation) the pre-condition - i.e. an identification of the inputs for which it will terminate and produce a result
(where this is possible to state, of course).

+1 from me too. The partiality of a function seems to me like something that should be documented.
Best,
Daniel
Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my experience, they will soon reach for functions like `head` and `tail`, because pattern-matching is foreign to them. I would love just to be able to say "Don't use partial functions", but many students will not easily be able to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then students will be able to identify partial functions, but loudly marking partial functions as partial seems like a small service to everyone and a bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
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--------------------------------------------------------------------
Andrew Butterfield Tel: [phone] Fax: [phone]
***@TCD, Head of Foundations & Methods Research Group
School of Computer Science and Statistics,
Room G.39, O'Reilly Institute, Trinity College, University of Dublin
http://www.scss.tcd.ie/Andrew.Butterfield/
--------------------------------------------------------------------

[Andrew Martin]

I'm strongly +1 on this.

Post by Richard Eisenberg
Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my
experience, they will soon reach for functions like `head` and `tail`,
because pattern-matching is foreign to them. I would love just to be able
to say "Don't use partial functions", but many students will not easily be
able to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then
students will be able to identify partial functions, but loudly marking
partial functions as partial seems like a small service to everyone and a
bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
_______________________________________________
Libraries mailing list
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

[Zemyla]

I feel like there's a difference between partial functions in the
sense of length and partial functions in the sense of head.

If you have a partiality monad like the free monad over Identity:

data Partial a = Done a | NotYet (Partial a)

instance Monad Partial where
return = Done
Done a >>= f = f a
NotYet m >>= f = NotYet $ m >>= f

then length has a sensible and productive implementation in terms of it:

partialLength :: [a] -> Partial Int
partialLength = go 0 where
go n ls = seq n $ case ls of
[] -> Done n
_:ls' -> NotYet $ go (n + 1) ls'

This is similar to the mechanism that Agda and Idris use to denote a
potentially non-terminating result; with it, these languages are
Turing complete.

head and tail aren't like that, and should be marked differently in
the documentation.

On Fri, Aug 31, 2018 at 7:28 AM, Andrew Martin

Post by Andrew Martin
I'm strongly +1 on this.

Post by Richard Eisenberg
Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my
experience, they will soon reach for functions like `head` and `tail`,
because pattern-matching is foreign to them. I would love just to be able to
say "Don't use partial functions", but many students will not easily be able
to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then
students will be able to identify partial functions, but loudly marking
partial functions as partial seems like a small service to everyone and a
bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
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--
-Andrew Thaddeus Martin
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[Richard Eisenberg]

I think we want something like "partial even given input you can successfully DeepSeq"

That's the specification of the feature I'm after. I think all the commentary about infinite lists, etc., would lead also to good documentation additions. (For example, it would be fantastic if every function precisely documented its strictness, preferably with some standard notation, but this is not the problem I'm trying to solve here.)

Also, it was suggested that the documentation be checked -- that is, we could imagine a {-# TOTAL ... #-} or {-# PARTIAL ... #-} pragma that GHC could check on compilation and Haddock could include in the documentation. This would also be great, but much more than I'm proposing here.

-- | /Contains a call to 'error'./ Extract the first element of a list, which must be non-empty.
head :: [a] -> a

In the end, it's the call to error that I want noted. Of course, having a similar note on functions like div (where the problem isn't a call to error) and length (that will loop on infinite lists) is good, but not really what I'm proposing here.

Thanks,
Richard

Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my experience, they will soon reach for functions like `head` and `tail`, because pattern-matching is foreign to them. I would love just to be able to say "Don't use partial functions", but many students will not easily be able to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then students will be able to identify partial functions, but loudly marking partial functions as partial seems like a small service to everyone and a bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
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Libraries mailing list
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[David Feuer]

Why isn't the call to error in div what you mean?

In a response not cc'd to the list, a contributor (not sure if they want

I think we want something like "partial even given input you can

successfully DeepSeq"
That's the specification of the feature I'm after. I think all the
commentary about infinite lists, etc., would lead also to good
documentation additions. (For example, it would be fantastic if every
function precisely documented its strictness, preferably with some standard
notation, but this is not the problem I'm trying to solve here.)
Also, it was suggested that the documentation be checked -- that is, we
could imagine a {-# TOTAL ... #-} or {-# PARTIAL ... #-} pragma that GHC
could check on compilation and Haddock could include in the documentation.
This would also be great, but much more than I'm proposing here.

-- | /Contains a call to 'error'./ Extract the first element of a list,

which must be non-empty.

head :: [a] -> a

In the end, it's the call to error that I want noted. Of course, having a
similar note on functions like div (where the problem isn't a call to
error) and length (that will loop on infinite lists) is good, but not
really what I'm proposing here.
Thanks,
Richard

Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my

experience, they will soon reach for functions like `head` and `tail`,
because pattern-matching is foreign to them. I would love just to be able
to say "Don't use partial functions", but many students will not easily be
able to tell partial functions from total ones.

I do expect this problem to work itself out rather quickly, and then

students will be able to identify partial functions, but loudly marking
partial functions as partial seems like a small service to everyone and a
bigger one to newbies. I don't see any downsides.

Thoughts?
Thanks,
Richard
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[Richard Eisenberg]

Because I was silly and didn't look for it. That can and should be included, yes.

Post by David Feuer
Why isn't the call to error in div what you mean?

I think we want something like "partial even given input you can successfully DeepSeq"

That's the specification of the feature I'm after. I think all the commentary about infinite lists, etc., would lead also to good documentation additions. (For example, it would be fantastic if every function precisely documented its strictness, preferably with some standard notation, but this is not the problem I'm trying to solve here.)
Also, it was suggested that the documentation be checked -- that is, we could imagine a {-# TOTAL ... #-} or {-# PARTIAL ... #-} pragma that GHC could check on compilation and Haddock could include in the documentation. This would also be great, but much more than I'm proposing here.

-- | /Contains a call to 'error'./ Extract the first element of a list, which must be non-empty.
head :: [a] -> a

In the end, it's the call to error that I want noted. Of course, having a similar note on functions like div (where the problem isn't a call to error) and length (that will loop on infinite lists) is good, but not really what I'm proposing here.
Thanks,
Richard

Proposal: Mark partial functions in `base` as partial
Motivation: I'm about to teach Haskell to a classful of beginners. In my experience, they will soon reach for functions like `head` and `tail`, because pattern-matching is foreign to them. I would love just to be able to say "Don't use partial functions", but many students will not easily be able to tell partial functions from total ones.
I do expect this problem to work itself out rather quickly, and then students will be able to identify partial functions, but loudly marking partial functions as partial seems like a small service to everyone and a bigger one to newbies. I don't see any downsides.
Thoughts?
Thanks,
Richard
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[Henrik Nilsson]

Hi,

-- | /Contains a call to 'error'./ Extract the first element of a

list, which must be non-empty.

head :: [a] -> a

In the end, it's the call to error that I want noted.

Yes, whether the code of a function directly or indirectly calls "error"
is well-defined property that could be documented.

I think we want something like "partial even given input you can successfully DeepSeq"

That's the specification of the feature I'm after

Consider e.g.

apply f x = f x

As e.g.

apply head [] == _|_

we'd have to conclude by the above definition that "apply" is partial.
But it clearly does not "call error".

Just to be clear, this does not correspond to how I understand
partiality. Lots of higher order functions, like "map", then would have
to called partial. And I am not sure that would be so helpful for
the purpose of alerting (new) programmers to functions that one
might argue should be avoided.

So maybe something very clear and easy to understand, such as "contains
call to error" is the best approach.

Best,

/Henrik



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[Ryan Trinkle]

This is probably not really for the libraries list, but in the same vein as
this discussion, I think it would be extremely helpful to have a way of
banning functions that does *not* require changing the upstream library. I
would be thrilled to have a machine-checked ban on use of the 'head',
'tail', etc.

On Fri, Aug 31, 2018 at 11:36 AM Henrik Nilsson <

Post by Henrik Nilsson
Hi,

-- | /Contains a call to 'error'./ Extract the first element of a

list, which must be non-empty.

head :: [a] -> a

In the end, it's the call to error that I want noted.

Yes, whether the code of a function directly or indirectly calls "error"
is well-defined property that could be documented.

I think we want something like "partial even given input you can
successfully DeepSeq"

That's the specification of the feature I'm after

Consider e.g.
apply f x = f x
As e.g.
apply head [] == _|_
we'd have to conclude by the above definition that "apply" is partial.
But it clearly does not "call error".
Just to be clear, this does not correspond to how I understand
partiality. Lots of higher order functions, like "map", then would have
to called partial. And I am not sure that would be so helpful for
the purpose of alerting (new) programmers to functions that one
might argue should be avoided.
So maybe something very clear and easy to understand, such as "contains
call to error" is the best approach.
Best,
/Henrik
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and may contain confidential information. If you have received this
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attachment.
Any views or opinions expressed by the author of this email do not
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communications with the University of Nottingham may be monitored
where permitted by law.
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[Henning Thielemann]

Post by Ryan Trinkle
This is probably not really for the libraries list, but in the same vein
as this discussion, I think it would be extremely helpful to have a way
of banning functions that does *not* require changing the upstream
library.  I would be thrilled to have a machine-checked ban on use of
the 'head', 'tail', etc.

So far I have HLint rules to ban them. Unfortunately, they do not work
transitively. People have already proposed a TotalHaskell pragma
analogously to SafeHaskell.

[Tikhon Jelvis]

Deeper questions of partiality aside, if a function can throw exceptions,
the documentation should spell this out *explicitly*, ideally also
specifying what the type of the exception may be. To me, this is a
*minimum* standard to expect from documentation in any language with
exceptions.

Whenever I write a function that might error out in my work code, I always
write a separate paragraph explaining this. Usually it reads something like
"Raises an error if these conditions aren't met" for whatever preconditions
the function has. I don't know if this documentation requirement is in our
style guide, but if it isn't I should put it in :).

This is definitely something we should have in the documentation for base.

Return the first element of the given list.
Raises an 'ErrorCall' exception if the list is empty.

I could see a case for putting less emphasis on the fact that it's a
specific exception—exceptions in pure functions are a bit weird—but the
basic structure would still be the same: a distinct part of the
documentation calling out the error condition explicitly.


On Fri, Aug 31, 2018, 17:54 Henning Thielemann <

Post by Ryan Trinkle
This is probably not really for the libraries list, but in the same vein
as this discussion, I think it would be extremely helpful to have a way
of banning functions that does *not* require changing the upstream
library. I would be thrilled to have a machine-checked ban on use of
the 'head', 'tail', etc.

So far I have HLint rules to ban them. Unfortunately, they do not work
transitively. People have already proposed a TotalHaskell pragma
analogously to SafeHaskell._______________________________________________
Libraries mailing list
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

[Oliver Charles]

Deeper questions of partiality aside, if a function can throw exceptions, the documentation should spell this out *explicitly*, ideally also specifying what the type of the exception may be. To me, this is a *minimum* standard to expect from documentation in any language with exceptions.

Is this to be applied transitively? It has to be, to meet your
criteria, but this is a fairly unrealistic expectation to put on
library authors. If it's not applied transitively, then the
documentation creates a false sense of security - "catch these
exceptions that *I* throw and you're good!". I really think this part
of exceptions is a tooling problem that we have to solve. Haddock
should be responsible for figuring this out.

Ollie

[Henning Thielemann]

Post by Tikhon Jelvis
Deeper questions of partiality aside, if a function can throw
exceptions, the documentation should spell this out *explicitly*,
ideally also specifying what the type of the exception may be. To me,
this is a *minimum* standard to expect from documentation in any
language with exceptions.

'error' is not an exception, it is an undefined value. 'error' is only a
debugging convenience that would otherwise be an infinite loop. It should
not be necessary to distinguish them in production code.

[Bardur Arantsson]

Post by Oliver Charles

Deeper questions of partiality aside, if a function can throw exceptions, the documentation should spell this out *explicitly*, ideally also specifying what the type of the exception may be. To me, this is a *minimum* standard to expect from documentation in any language with exceptions.

Is this to be applied transitively? It has to be, to meet your
criteria, but this is a fairly unrealistic expectation to put on
library authors. If it's not applied transitively, then the
documentation creates a false sense of security - "catch these
exceptions that *I* throw and you're good!". I really think this part
of exceptions is a tooling problem that we have to solve. Haddock
should be responsible for figuring this out.

Not to mention the issues of "may throw" declarations for higher-order
functions... (Java checked exceptions are basically broken[1] because of
this. Well, this + the variance issues arising from inheritance and
method signatures, but obviously Haskell doesn't have inheritance, so...)

Regards,

[1] They even break "catch" because everybody is forced to wrap
"checked" exceptions in a RuntimeException if signatures don't specify
"may throw anything", so you *also* have to check for wrapped exceptions
in "catch" clauses. Anyway...

[Tikhon Jelvis]

This is a question of documentation, not type design. You don't have to
consider what exceptions your transitive dependencies might throw—you need
to understand what inputs *do* cause your function to fail. If you use head
in a safe way or have a call to error in an unreachable case, you shouldn't
say anything. On the other hand, if your function fails when the inputs
aren't relatively prime, you should document this regardless of whether the
failure is an exception you throw explicitly, a result of a partial pattern
match or a division by zero four layers of indirection away.

I definitely expect the author of a function to understand what inputs
cause that function to fail. If you don't understand this, you simply don't
understand the code you wrote. (Luckily, QuickCheck will probably find any
edge cases you missed when writing the function.)

Thinking about it a bit more, there's nothing specific to exceptions here.
If your code loops forever when the lengths of the inputs sum to a multiple
of seven, that's pretty handy to know from the documentation!

I would also expect the same style of documentation even for errors
expressed in the types with whatever your favorite generalization of Either
happens to be.

Post by Oliver Charles

Post by Tikhon Jelvis
Deeper questions of partiality aside, if a function can throw

exceptions, the documentation should spell this out *explicitly*, ideally
also specifying what the type of the exception may be. To me, this is a
*minimum* standard to expect from documentation in any language with
exceptions.

Post by Oliver Charles
Is this to be applied transitively? It has to be, to meet your
criteria, but this is a fairly unrealistic expectation to put on
library authors. If it's not applied transitively, then the
documentation creates a false sense of security - "catch these
exceptions that *I* throw and you're good!". I really think this part
of exceptions is a tooling problem that we have to solve. Haddock
should be responsible for figuring this out.

Not to mention the issues of "may throw" declarations for higher-order
functions... (Java checked exceptions are basically broken[1] because of
this. Well, this + the variance issues arising from inheritance and
method signatures, but obviously Haskell doesn't have inheritance, so...)
Regards,
[1] They even break "catch" because everybody is forced to wrap
"checked" exceptions in a RuntimeException if signatures don't specify
"may throw anything", so you *also* have to check for wrapped exceptions
in "catch" clauses. Anyway...
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[Bardur Arantsson]

Post by Tikhon Jelvis
This is a question of documentation, not type design. You don't have to
consider what exceptions your transitive dependencies might throw—you
need to understand what inputs *do* cause your function to fail. If you
use head in a safe way or have a call to error in an unreachable case,
you shouldn't say anything. On the other hand, if your function fails
when the inputs aren't relatively prime, you should document this
regardless of whether the failure is an exception you throw explicitly,
a result of a partial pattern match or a division by zero four layers of
indirection away.
I definitely expect the author of a function to understand what inputs
cause that function to fail. If you don't understand this, you simply
don't understand the code you wrote. (Luckily, QuickCheck will probably
find any edge cases you missed when writing the function.)
Thinking about it a bit more, there's nothing specific to exceptions
here. If your code loops forever when the lengths of the inputs sum to a
multiple of seven, that's pretty handy to know from the documentation! 
I would also expect the same style of documentation even for errors
expressed in the types with whatever your favorite generalization of
Either happens to be.

Oh, of course, these are good points. My point was simply that it's
often a little bit complicated to *document* this in a simple easy to
understand way.

Regards,