Re: RFC: Add HasCallStack constraint to partial Data.List functions.

But nothing prevents us from agreeing on restricting the semantics of  `IsPartial` ! We can write down in the Haddocks “`IsPartial` should exclusively be used for functions that use error/errorWithoutStackTrace, and not for imprecise exceptions or asynchronous exceptions.” We have documentation, guides, tutorials, comments, style guides, etc. I just don't understand why it is a problem. And again, this is not a discussion that is unique to us. The PureScript community have had their own Partial typeclass for ages and this hasn't triggered the end of the world. We don't have to delve into year-long arguments on The Nature of Partiality. --- rant --- As a general comment, it is honestly appalling how RFCs like that trigger such circlejerks that are so utterly disconnected from reality. This does not make us better as a community. The last time I suggested a change in the base library (up-streaming strictness optimisations that were enabled at the first level of optimisation), I've was told that this went against the spirit of the language, even though a simple look at the most minor optimisation level would reveal that we haven't been "true" to this "spirit" for a long, long time. And this is extremely bizarre since we try to take the highest moral stances on some topics whilst making dirty compromises on other topics (for which I am not seeing anyone actively try to change the status quo).

It's been well known for close to three decades(!) that it is perfectly possible to add sophisticated debugging support to lazy languages like Haskell without impacting on the language semantics or formulation of its libraries. The existing tracing and profiling mechanisms of GHC are examples of this.

Henrik, Debug.Trace functions requires the use of "unsafePerformIO". Is this really the sort of thing we want to promote when we talk about "sophisticated debugging support to lazy languages without impacting the semantics"? Looks like the semantics of pure functional programming are flushed down the toilets with that one. Regarding profiling, we are *unable* to teach how it works. The Haskell Wiki article¹ is a stub, there are issues² on the Cabal bug tracker to ask how to enable it, and for most people it's still a mystery. So before we say things like "It's been well known for close to three decades(!)(sic)", I suggest we get our shit together because right now we look like a bunch of clowns. And if by any chance the University of Nottingham has a secret vault of forbidden techniques regarding profiling Haskell applications, I'd love that the rest of the community could benefit from this 30yo knowledge, since apparently nothing has filtered and we are still like monkey playing with sticks in the outside world. So let's meditate on this a bit, shall we? As much as I am not the biggest fan of ad-hoc typeclasses to signal operational behaviour, the inability of the community to promote the "better" ways to handle such failure modes makes me think that we have no right to spit on this RFC. --- ¹ https://wiki.haskell.org/How_to_profile_a_Haskell_program ² https://github.com/haskell/cabal/issues/5930 Le 09/06/2021 à 09:57, Julian Ospald a écrit :

I'm aware what was meant, but this was in response to the proposed 'IsPartial' typeclass, which will have this confusion pop up more often.

On June 9, 2021 7:48:58 AM UTC, "Hécate" wrote:

Let's not diverge too much on the nature of IO functions, in the context of this conversation, here are the functions that are partial:

- head, tail, init, last, ... - foldr1, foldl1, maximum, minimum, ... - (!!)

They all have in common the usage of `error(WithoutStackTrace)`, so we can safely say that 'partial ~ using error(WithoutStackTrace)'  in the context of this RFC.

Le 09/06/2021 à 09:31, Julian Ospald a écrit :

...

Yeah, I think a typeclass to express partiality is a sloppy technique. I'm not even sure everyone agrees on what 'partial' means: if my function throws an IO error on relative FilePaths, is it partial? Is all IO partial?

Haskell is definitely not a total language and I doubt it will be. I also don't think that it's an interesting goal. It requires a considerable shift in language design.

HasCallstack sounds like a pragmatic solution, but you could as well create an alternative prelude that adds it everywhere you want and then avoid implicit prelude. That won't help you with unsound dependencies, that don't use it, but it's opt-in, which seems more reasonable given that it's obviously a somewhat controversial change.

I'd expect the nay-sayers here, however, to be a driving force in a better GHC based solution. Otherwise, the next time this comes up people will say "you had time enough".

On June 8, 2021 6:10:52 PM UTC, Oliver Charles wrote:

On Tue, 8 Jun 2021, at 6:36 PM, Richard Eisenberg wrote:

...

I've been very much of two minds in this debate: On the one hand, having these constraints is very practically useful. On the other, what we're doing here is very un-Haskellish, in that we're letting operational concerns leak into a declarative property (a function's type). The reason we're doing this is another un-Haskellish thing -- partiality -- but that ship has sailed.

So, may I propose a slightly different way forward?

Instead of adding a HasCallStack constraint on these functions, add an IsPartial constraint. For example:

> head :: IsPartial => [a] -> a

This is slightly awkward, still, because IsPartial is a class-constraint-like-thing, but it has no parameter. But it has a few very nice properties: * IsPartial is declarative: it describes a property of the function without worrying about its operation. * If we think about the way constraints propagate, IsPartial has the right semantics: the caller of a partial function would itself become partial.

I don't think this is true.

Take:

foo :: Int -> Bool foo _ = head [True]

Clearly foo is total - it is defined for all of its inputs. That it uses a partial function in its body isn't observable. So it's a shame that IsPartial leaks out.

I guess here you'd have me say

foo _ = partialityIsOk $ head [True]

?

Ollie

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