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<nij_>
One thing I find weird about CL type system is the SATISFIES keyword. It doesn't seem to support sane interface. Can I safely assume that the "types" mentioned in CL is not as what other people mean usually?
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<nij->
Ooops my first message didn't send through: > I've always been confused with the difference between classes and types. Until today, I read [1] which seems to make it click. My summary is that classes are moer about implementation details, while types are more about their interfaces. That's maybe why types support polymorphisms with more sanity. [1]
<ixelp>
Differences between subtypes and subclasses
<pjb>
nij-: first thing to note, is that there's no first class types in Common Lisp (there's no type type). What we have are type DESIGNATORS, mere sexps denoting types.
<pjb>
nij-: there's a little exception, for classes, since classes are type designators for the set of their instances, so in a way, class objects are first class types, but it's an exceptionnal situation, and #<class foo> is equivalent to foo as type designators.
<Bike>
nij-: lisp types are different from types in most languages (and e.g. type theory) because they are quite explicitly sets, and moreover sets in naive set theory
<pjb>
nij-: Well, types in CL forms a lattice. This is not always the case. They represent indeed sets of values, but all types should be sets of values.
<pjb>
satisfies just takes a predicate. This (satisfies foop) = { x | foop(x) } which is perfectly defined set, therefore a perfectly defined type.
<pjb>
well, it takes the name of a predicate.
<Alfr>
self-not-included-p
<pjb>
(defun self-not-included-p (x) (not (member x x))) (self-not-included-p '(a b c)) #| --> t |# (self-not-included-p '#1=(a b #1# d)) #| --> nil |#
<pjb>
(deftype self-not-included () '(satisfies self-not-included-p)) (values (typep '(a b c) 'self-not-included) (typep '#1=(a b #1# d) 'self-not-included)) #| --> t ; nil |#
<nij_>
I see. Thanks. I understand those as "CL types are more like sets, which differ from the types in most languages."
<Alfr>
pjb, oh, no. I didn't mean that you can't define it, but that the result isn't a set.
<nij_>
I wonder how types are implemented then.. can they be implemented by the users?
<pjb>
nij_: the trick is that in lisp, it's values that have types, not variables. In languages like C, you declare the type of the variables, the type of boxes. But you can put any values in those variables (any bit pattern). In Lisp, all the boxes, all the variables can hold any value, each value having some types. (a whole lattice of types in general).
<pjb>
nij_: of course, you can.
<pjb>
Just like a compiler writter.
<pjb>
For example, 42 is of type (integer 42 42), fixnum, (integer 0), (integer -1 333), of type (satisfies evenp), (member 21 33 42), (eql 42), etc…
<pjb>
and also integer, ratio, real, number and t.
<pjb>
So it would be difficult to restrict boxes to a single type…
<pjb>
difficult and rather meaningless.
<nij_>
Oh.. I should rephrase "I wonder how *usual* (i.e. non-CL) types are implemented then.. can they be implemented by the users?"
<pjb>
Note also that in lattices, there are elements that are not comparables (sets that are not subset one of the other).
<Bike>
nij-: that's classes, probably
<jcowan>
More broadly considered, in statically typed languages, types are the types of expressions, not only variables.
<Bike>
lisp types are attached to values rather than expressions or variables, which is part of why they're different
<nij->
Bike: what did you mean by "that" in "that's classes".
<Bike>
"non-cl" types in lisp
<Bike>
each value has exactly one direct class
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<pjb>
At least at one time.
<pjb>
Because with change-class, a given object may have several different direct classes, at different times.
<aeth>
But if you do things that are so dynamic that they might break things, that's on you... if someone CHECK-TYPEs and you get around that, well, you're probably now breaking the assumptions of the program.
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<aeth>
CL is powerful enough to let you shoot yourself in the foot.
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<beach>
nij-: It is types in other languages that are strange and complicated, yet restricted. They must be so that types can be determined statically. And they don't even succeed very well. Determining the exact type of an expression is undecidable in general, so those languages do what they can to make it decidable.
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<pve>
Hey! Does "hook" refer to the variable to which functions (or symbols) are added or does it refer to the *functions* that are added to the list variable? I think I've seen it used both ways.
<pjb>
pve: in general, to both. But I use meat for the functions, so I can hang meat on the hooks. ;-)
<pve>
I have a list of symbols that name functions, and considered calling it a hook
<pve>
haha
<pjb>
pve: yes, the list is definitely a hook if the functions are called at specific point to allow for extensions.
<pjb>
(the variable holding the list)
<pve>
great, thanks
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<prxq>
any mito users here?
<prxq>
I'm wondering how to deal with the connection. ORMs in other languages have "sessions", which reconnect when the server times out. Mito does not seem to have something like it.
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<pjb>
Then you'd need to implement such session layer. That said, with modern network hardware, TCP is usually stable, it's rare that you get disconnected.
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<prxq>
pjb: mysql actually times out.
<prxq>
pjb: ok
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<prxq>
i'm having trouble searching in tables for items having NULL values. Basically, SELECT column_names FROM table_name WHERE column_name IS NULL; Anyone knows how to specify this?
<prxq>
It sucks also that I can't see the sql :-) somehow it's not printing it.
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<prxq>
so no, you can't search for elements having a null field. That sucks. Very disappointing.
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<bitspook[m]>
prxq: Is it not possible to use sxql to create mito objects?
<prxq>
bitspook[m]: yeah but even there I'm not so sure. How'd you write a sxql search for a row with a null value?
<bitspook[m]>
I suppose so. I haven't had a problem with building an SQL query with sxql so far; if worse comes to worse, you can write raw SQL.
<pjb>
prxq: my advice would be to drop mysql and to use a real RDBMS. Try Postgres.
<prxq>
pjb: this has nothing to do with that.
<prxq>
would be the exact same problem on postgres or sqlite.
<bitspook[m]>
prxq: Above is one way to do it without fighting sxql/mito too much. I am not sure what you're trying to do, but maybetry `(:raw "PART OF QUERY IN RAW SQL")`
<pjb>
selecting null columns should work.
<prxq>
bitspook[m]: thanks, that looks good!
<prxq>
pjb: the problem is getting the right sql query out of the orm, Once the sql is correct, it finds them just fine
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