### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

I wrote: What goes wrong with finite coproducts? The obvious thing to do would be to take the disjoint union of the sets representing the types, identifying the copies of _|_. Jonathan Cast wrote: This isn't a coproduct. If we have f x = 1 and g y = 2, then there should exist a function h such that h . Left = f and h . Right = g... But by your rule, Left undefined = Right undefined... Which is a contradiction... OK, thanks. Unless, of course, your require your functions to be strict --- then both f and g above become illegal, and repairing them removes the problem. You don't have to make them illegal - just not part of your notion of coproduct. That is an entirely category-theoretic concept, since Empty is bi-universal, and a morphism is strict iff the diagram f A --- B \ ^ v/ Empty commutes. However, the coproduct you get is the one I suggested, namely Either !A !B, not the one we usually use. What is the lifted version you are referring to? Take the ordinary disjoint union, and then add a new _|_ element, distinct from both existing copies of _|_ (which are still distinct from each other). Of course, Haskell makes things even worse by lifting the product and exponential objects, OK, what goes wrong there, and what is the lifting? Again, in Haskell, (_|_, _|_) /= _|_. The difference is in the function f (x, y) = 1 which gives f undefined = undefined but f (undefined, undefined) = 1. Unfortunately, this means that (alpha, beta) has an extra _|_ element (_|_, _|_), so it's not the categorical product (which would lack such an element for the same reason as above). This is partly an implementation issue --- compiling pattern matching without introducing such a lifting requires a parallel implementation --- and partly a semantic issue --- data introduces a single level of lifting, every time it is used, and every constructor is completely lazy. Functions have the same issue --- in the presence of seq, undefined / = const undefined. Extensionality is a key part of the definition of all of these constructions. The categorical rules are designed to require, in concrete categories, that the range of the two injections into a coproduct form a partition of the coproduct, the surjective pairing law (fst x, snd x) = x holds, and the eta reduction law (\ x - f x) = f holds. Haskell flaunts all three; while some categories have few enough morphisms to get away with this (at least some times), Hask is not one of them. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

On Wed, 2008-01-02 at 15:49 +0200, Yitzchak Gale wrote: [...] Some people are worried that this version of Hask is missing certain nice properties that one would like to have. For example, it was recently claimed on this list that tuples are not products in that category. (Or some such. I would be interested to see a demonstration of that.) Johnathan has given such a demonstration (and it has been demonstrated many times on this list since it's creation, it's well-known). I am not impressed by those complaints. As usual in category theory, you define corresponding notions in Hask, and prove that they are preserved under the appropriate functors. That should always be easy. And if ever it is not, then you have discovered an interesting non-trivial consequence of laziness that deserves study. You are right not to be impressed by such complaints, but you misrepresent people's views on this by saying that the worry about such problems. As you say (people say), these properties [that Hask is cartesian closed to start] would be nice to have and are very convenient to assume which is often safe enough. Certainly computer scientists of a categorical bent have developed (weaker) notions to use; namely, monoidal, pre-monoidal, Freyd, and/or kappa categories and no doubt others. Using these, however, removes some of the allure of using a categorical approach. Also, there is a Haskell-specific problem at the very get-go. The most obvious choice for the categorical composition operator assuming the obvious choice for the arrows and objects does not work, it does not satisfy the laws of a category assuming the = used in them is observational equality. Namely, id . f /= f /= f . id for all functions f, in particular, it fails when f = undefined. This can easily be fixed by making the categorical (.) strict in both arguments and there is no formal problem with it being different from Haskell's (.), but it certainly is not intuitively appealing. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

(sorry, I hit the send button) What is the lifted version you are referring to? Take the ordinary disjoint union, and then add a new _|_ element, distinct from both existing copies of _|_ (which are still distinct from each other). Now why is that not the category-theoretic coproduct? h . Left = f and h . Right = g both for _|_ and for finite elements of the types. And it looks universal to me. Of course, Haskell makes things even worse by lifting the product and exponential objects, OK, what goes wrong there, and what is the lifting? Again, in Haskell, (_|_, _|_) /= _|_. The difference is in the function f (x, y) = 1 which gives f undefined = undefined but f (undefined, undefined) = 1. I don't get that one. Given any f and g, we define h x = (f x, g x). Why do we not have fst . h = f and snd . h = g, both in Hask and StrictHask? fst and snd are strict. Unfortunately, this means that (alpha, beta) has an extra _|_ element (_|_, _|_), so it's not the categorical product (which would lack such an element for the same reason as above). The reason you can't adjoin an extra element to (A,B) in, say, Set, is that you would have nowhere to map it under fst and snd. But here that is not a problem, _|_ goes to _|_ under both. This is partly an implementation issue --- compiling pattern matching without introducing such a lifting requires a parallel implementation That's interesting. So given a future platform where parallelizing is much cheaper than it is today, we could conceivably have a totally lazy version of Haskell. I wonder what it would be like to program in that environment, what new problems would arise and what new techniques would solve them. Sounds like a nice research topic. Who is working on it? --- and partly a semantic issue --- data introduces a single level of lifting, every time it is used, and every constructor is completely lazy. Unless you use bangs. So both options are available, and that essentially is what defines Haskell as being a non-strict language. Functions have the same issue --- in the presence of seq, undefined / = const undefined. Right. I am becoming increasingly convinced that the seq issue is a red herring. Extensionality is a key part of the definition of all of these constructions. The categorical rules are designed to require, in concrete categories, that the range of the two injections into a coproduct form a partition of the coproduct, the surjective pairing law (fst x, snd x) = x holds, and the eta reduction law (\ x - f x) = f holds. Haskell flaunts all three; while some categories have few enough morphisms to get away with this (at least some times), Hask is not one of them. That interpretation is not something that is essential in the notion of category, only in certain specific examples of categories that you know. Product and coproduct in any given category - whether they exist, what they are like if they exist, and what alternative constructions exist if they do not - reflect the nature of the structure that the category is modeling. I am interested in understanding the category Hask that represents what Haskell really is like as a programming language. Not under some semantic mapping that includes non-computable functions, and that forgets some of the interesting structure that comes from laziness (though that is undoubtably also very interesting). -Yitz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

I wrote: ...it was recently claimed on this list that tuples are not products in that category. Derek Elkins wrote: Johnathan has given such a demonstration (and it has been demonstrated many times on this list since it's creation, it's well-known). We're still working on it. I've not been convinced yet. Sorry about my thickness. Perhaps this should be on a nice wiki page somewhere. I tried to convince David House to put it in the Wikibook chapter, but he is right, that needs to simpler. There is some discussion on the talk page for that chapter, but no one spells out the details there, either. You are right not to be impressed by such complaints, but you misrepresent people's views on this by saying that they worry about such problems. Sorry, I hope I am not misrepresenting anyone. I just notice that people make assumptions about a so-called category Hask, derive various conclusions, then mention that they are not really true. Perhaps it is only me who is worried. As you say (people say), these properties [that Hask is cartesian closed to start] would be nice to have and are very convenient to assume which is often safe enough. I'd like to understand better what is true, so that I can understand what is safe. Certainly computer scientists of a categorical bent have developed (weaker) notions to use; namely, monoidal, pre-monoidal, Freyd, and/or kappa categories and no doubt others. Using these, however, removes some of the allure of using a categorical approach. It would be nice to distill out of that the basics that are needed to get the properties that we need for day-to-day work in Haskell. Also, there is a Haskell-specific problem at the very get-go. The most obvious choice for the categorical composition operator assuming the obvious choice for the arrows and objects does not work... ...This can easily be fixed by making the categorical (.) strict in both arguments and there is no formal problem with it being different from Haskell's (.), but it certainly is not intuitively appealing. I'm not sure it's so bad. First of all, not only is it not a formal problem, it's also not really a practical problem - there will rarely if ever be any difference between the two when applied in real programs. My opinion is that there would not be any problem with using that version as (.) in the Prelude. Even if we never do, at least we should then use (.!) when we state the so-called Monad laws. It bothers me that Haskell's so-called monads really aren't. That is bound to cause problems. And it would be so easy to fix in most cases - just require monad bind to be strict on the second parameter. Thanks, Yitz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

Take the ordinary disjoint union, and then add a new _|_ element, distinct from both existing copies of _|_ (which are still distinct from each other). Now why is that not the category-theoretic coproduct? h . Left = f and h . Right = g both for _|_ and for finite elements of the types. And it looks universal to me. Yeah, but there could be more functions from Either X Y to Z than pairs of functions from X to Z and from Y to Z. For example, if z :: Z, then you have two functions h1 and h2 such that h1 . Left = const z and h1 . Right = const z and the same holds for h2. Namely, h1 = const z h2 = either (const z) (const z) This functions are different : h1 (_|_) = z while h2 (_|_) = (_|_). And if Either X Y was a category-theoretic coproduct, then the function from Either X Y to Z would be UNIQUELY determined by it's restrictions to X and Y. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

On 6 Jan 2008, at 3:55 AM, Yitzchak Gale wrote: I wrote: What goes wrong with finite coproducts? The obvious thing to do would be to take the disjoint union of the sets representing the types, identifying the copies of _|_. Jonathan Cast wrote: This isn't a coproduct. If we have f x = 1 and g y = 2, then there should exist a function h such that h . Left = f and h . Right = g... But by your rule, Left undefined = Right undefined... Which is a contradiction... OK, thanks. Unless, of course, your require your functions to be strict --- then both f and g above become illegal, and repairing them removes the problem. You don't have to make them illegal - just not part of your notion of coproduct. We're talking past each other --- what is the distinction you are making? jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

On 6 Jan 2008, at 5:32 AM, Yitzchak Gale wrote: (sorry, I hit the send button) What is the lifted version you are referring to? Take the ordinary disjoint union, and then add a new _|_ element, distinct from both existing copies of _|_ (which are still distinct from each other). Now why is that not the category-theoretic coproduct? h . Left = f and h . Right = g both for _|_ and for finite elements of the types. And it looks universal to me. Not quite. The only requirement for h _|_ is that it be = f x for all x and = g y for all y. If f x = 1 = g y for all x, y, then both h _|_ = _|_ and h _|_ = 1 are arrows of the category. So the universal property still fails. Of course, Haskell makes things even worse by lifting the product and exponential objects, OK, what goes wrong there, and what is the lifting? Again, in Haskell, (_|_, _|_) /= _|_. The difference is in the function f (x, y) = 1 which gives f undefined = undefined but f (undefined, undefined) = 1. I don't get that one. Given any f and g, we define h x = (f x, g x). Why do we not have fst . h = f and snd . h = g, both in Hask and StrictHask? fst and snd are strict. Again, if f and g are both strict, we have a choice for h _|_ --- either h _|_ = _|_ or h _|_ = (_|_, _|_) will work (fst . h = f and snd . h = g), but again these are different morphisms. Unfortunately, this means that (alpha, beta) has an extra _|_ element (_|_, _|_), so it's not the categorical product (which would lack such an element for the same reason as above). The reason you can't adjoin an extra element to (A,B) in, say, Set, is that you would have nowhere to map it under fst and snd. But here that is not a problem, _|_ goes to _|_ under both. This is partly an implementation issue --- compiling pattern matching without introducing such a lifting requires a parallel implementation That's interesting. So given a future platform where parallelizing is much cheaper than it is today, we could conceivably have a totally lazy version of Haskell. I wonder what it would be like to program in that environment, what new problems would arise and what new techniques would solve them. Sounds like a nice research topic. Who is working on it? --- and partly a semantic issue --- data introduces a single level of lifting, every time it is used, and every constructor is completely lazy. Unless you use bangs. So both options are available, and that essentially is what defines Haskell as being a non-strict language. (!alpha, !beta) isn't a categorical product, either. snd (undefined, 1) = undefined with this type. Functions have the same issue --- in the presence of seq, undefined / = const undefined. Right. I am becoming increasingly convinced that the seq issue is a red herring. Care to give an explanation? Extensionality is a key part of the definition of all of these constructions. The categorical rules are designed to require, in concrete categories, that the range of the two injections into a coproduct form a partition of the coproduct, the surjective pairing law (fst x, snd x) = x holds, and the eta reduction law (\ x - f x) = f holds. Haskell flaunts all three; while some categories have few enough morphisms to get away with this (at least some times), Hask is not one of them. That interpretation is not something that is essential in the notion of category, only in certain specific examples of categories that you know. I understand category theory. I also know that the definitions used are chosen to get Set `right', which means extensionality in that case, and are then extended uniformly across all categories. This has the effect of requiring similar constructions to those in Set in other concrete categories. Product and coproduct in any given category - whether they exist, what they are like if they exist, and what alternative constructions exist if they do not - reflect the nature of the structure that the category is modeling. I understand that. I'm not sure you do. I am interested in understanding the category Hask that represents what Haskell really is like as a programming language. Good luck. Not under some semantic mapping that includes non-computable functions, and that forgets some of the interesting structure that comes from laziness (though that is undoubtably also very interesting). Bear in mind in your quest, that at the end of it you'll most likely conclude, like everyone else, that good old equational reasoning is sound for the programs you actually right at least 90% of the time (with appropriate induction principles), and complete for at least 90% of what you want to right, and go back to using it exclusively for real programming. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

Jonathan Cast wrote: The normal view taken by Haskellers is that the denotations of Haskell types are CPPOs. So: (1) Must be monotone (2) Must be continuous (Needn't be strict, even though that messes up the resulting category substantially). I wrote: I'm not convinced that the category is all that messed up. Well, no coproducts (Haskell uses a lifted version of the coproduct from CPO). What goes wrong with finite coproducts? The obvious thing to do would be to take the disjoint union of the sets representing the types, identifying the copies of _|_. What is the lifted version you are referring to? Of course, Haskell makes things even worse by lifting the product and exponential objects, OK, what goes wrong there, and what is the lifting? Thanks, Yitz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

On 5 Jan 2008, at 6:03 PM, Yitzchak Gale wrote: Jonathan Cast wrote: The normal view taken by Haskellers is that the denotations of Haskell types are CPPOs. So: (1) Must be monotone (2) Must be continuous (Needn't be strict, even though that messes up the resulting category substantially). I wrote: I'm not convinced that the category is all that messed up. Well, no coproducts (Haskell uses a lifted version of the coproduct from CPO). What goes wrong with finite coproducts? The obvious thing to do would be to take the disjoint union of the sets representing the types, identifying the copies of _|_. This isn't a coproduct. If we have f x = 1 and g y = 2, then there should exist a function h such that h . Left = f and h . Right = g, i.e., h (Left x) = f x = 1 and h (Right y) = g y = 2 But by your rule, Left undefined = Right undefined, so 1 = h (Left undefined) = h (Right undefined) = 2 Which is a contradiction. Identifying Left _|_ and Right _|_ produces a pointed CPO, but it's not a coproduct. Unless, of course, your require your functions to be strict --- then both f and g above become illegal, and repairing them removes the problem. What is the lifted version you are referring to? Take the ordinary disjoint union, and then add a new _|_ element, distinct from both existing copies of _|_ (which are still distinct from each other). Of course, Haskell makes things even worse by lifting the product and exponential objects, OK, what goes wrong there, and what is the lifting? Again, in Haskell, (_|_, _|_) /= _|_. The difference is in the function f (x, y) = 1 which gives f undefined = undefined but f (undefined, undefined) = 1. Unfortunately, this means that (alpha, beta) has an extra _|_ element (_|_, _|_), so it's not the categorical product (which would lack such an element for the same reason as above). This is partly an implementation issue --- compiling pattern matching without introducing such a lifting requires a parallel implementation --- and partly a semantic issue --- data introduces a single level of lifting, every time it is used, and every constructor is completely lazy. Functions have the same issue --- in the presence of seq, undefined / = const undefined. Extensionality is a key part of the definition of all of these constructions. The categorical rules are designed to require, in concrete categories, that the range of the two injections into a coproduct form a partition of the coproduct, the surjective pairing law (fst x, snd x) = x holds, and the eta reduction law (\ x - f x) = f holds. Haskell flaunts all three; while some categories have few enough morphisms to get away with this (at least some times), Hask is not one of them. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

Hi Jonathan, I wrote: So in what way are Set morphisms restricted from being Hask morphisms? Jonathan Cast wrote: The normal view taken by Haskellers is that the denotations of Haskell types are CPPOs. CPPO? So: (1) Must be monotone (2) Must be continuous Could you please define what you mean by those terms in this context? (Needn't be strict, even though that messes up the resulting category substantially). I'm not convinced that the category is all that messed up. Thanks, Yitz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

Jonathan Cast wrote: The normal view taken by Haskellers is that the denotations of Haskell types are CPPOs. I wrote: CPPO? (1) Must be monotone (2) Must be continuous Could you please define what you mean by those terms in this context? Jens Blanck wrote: The extra P would stand for pointed (has a least element, bottom), this is common in some communities. To me though, a cpo (complete partial order) is closed under directed suprema and the empty set is directed so bottom is already required. The category of cpos in not cartesian closed. For denotational semantics I believe the subcategory of Scott domains are what is usually considered. Continuous functions on cpos are by definition monotone and they respect directed suprema. Thanks! Yitz ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

On 3 Jan 2008, at 3:40 AM, Jens Blanck wrote: The normal view taken by Haskellers is that the denotations of Haskell types are CPPOs. CPPO? So: (1) Must be monotone (2) Must be continuous Could you please define what you mean by those terms in this context? (Needn't be strict, even though that messes up the resulting category substantially). I'm not convinced that the category is all that messed up. Well, no coproducts (Haskell uses a lifted version of the coproduct from CPO). Of course, Haskell makes things even worse by lifting the product and exponential objects, as well, which come to think of it is unnecessary even in the category of CPPOs and not necessarily strict continuous functions. The extra P would stand for pointed (has a least element, bottom), this is common in some communities. To me though, a cpo (complete partial order) is closed under directed suprema and the empty set is directed so bottom is already required. Not so. A set is directed iff every finite subset has an upper bound in the set; {} is finite, so it must have an upper bound in the set. So directed sets must be non-empty. (So CPOs needn't be pointed). jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

### Re: [Haskell-cafe] Basic question concerning the category Hask (was: concerning data constructors)

On 2 Jan 2008, at 5:49 AM, Yitzchak Gale wrote: Hi Andrew, Andrew Bromage wrote: I still say it isn't a set in the same way that a group isn't a set. Haskell data types have structure that is respected by Haskell homomorphisms. Sets don't. Ah, that's certainly true. But what is that additional structure? In categories that have a forgetful functor to Set, the interesting part of their structure comes from the fact that their morphisms are only a proper subset of the morphisms in Set. So in what way are Set morphisms restricted from being Hask morphisms? The normal view taken by Haskellers is that the denotations of Haskell types are CPPOs. So: (1) Must be monotone (2) Must be continuous (Needn't be strict, even though that messes up the resulting category substantially). jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe