<Dairyfarm>
But ffmpeg, which was what I wanted to use with liquidsoap won't compile.
<Dairyfarm>
Awesome, I don't think I needed the ffmpeg plugin anyways.
<Dairyfarm>
Thanks tizoc.
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<tizoc>
Dairyfarm: cool, glad it worked
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<hdon>
hi all :) is it possible to index collections by constructors? in C when using tagged unions (which aren't type-safe and require programmer discipline to ensure they are used safely) my tags (using enums) are also integers, which means i can use them as keys for arrays or other kinds of collections. further, i can count them by simply adding a tag like TAG_COUNT as the last item in my enum and perform
<hdon>
arithmetic on the cardinality of my types which is useful for dimensioning collections whose storage requirements or shapes are related to the cardinality of my types. any advice here for an ocaml newb?
<hdon>
a contrived example: suppose my students follow a dress code. shirts can be black or white, pants can be black, white, or beige. i want a representation of outfit frequency with O(1) access. pseudo code: enum Pants { Black, White, Beige, PantsMax }; enum Shirts { Black, White, ShirtsMax }; int outfitFreq[PantsMax*ShirtsMax]; int get(shirt, pants) { return outfitFreq[pants+shirt*PantsMax]; }
<hdon>
one solution could be to write converter functions: type shirt = BlackShirt | WhiteShirt; type pants = BlackPants | WhitePants | BeigePants; let shirtColorCount = 2; let pantsColorCount = 3; let shirtToInt = (x:shirt) = fn | WhiteShirt => 0 | BlackShirt => 1; let pantsToInt (x:pants) = fn | WhitePants => 0 | BlackPants => 1 | BeigePants => 2; /* etc */
<hdon>
but that means more typing and room for mistakes :)
<pierpa>
You could use a hash table indexed by (shirt,pants)
<pierpa>
no need to convert to an integer index
<hdon>
pierpa: hash table doesn't have O(log(n)) lookup time and hashing overhead?
<pierpa>
no
<hdon>
oh cool
<pierpa>
hash tables are O(1). That's the their point :)
<hdon>
so, IIUC, lookup time with this will be nearly constant, because buckets will tend to hold no more than one key-value pair?
<pierpa>
more or less. To be precise, lookup time is O(1).
<pierpa>
in practice, it is fast enough, you don't need to worry.
<hdon>
can constructors place limits on the values of their arguments beyond their number and types?
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<pierpa>
AFAIK, no.
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<marxS>
hi
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<kakadu>
Hey, Drup. Which version of z3 should I have to link with z3overlay?
* kakadu
has some linking errors using z3 4.4.1
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<Drup>
kakadu: hmm, can you report ?
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<kakadu>
Drup: I actually think it is not a problem of z3overlay but a problem of z3 opam package. Don't worry, it seems that it should be a problem companion_cube :) https://github.com/ocaml/opam-repository/issues/11882
<kakadu>
of*
<companion_cube>
I suppose it's a problem of LD_LIBRARY_PATH
<kakadu>
why?
<companion_cube>
ah wait
<companion_cube>
the z3 package on opam comes with its own z3
<companion_cube>
afair it doesn't use the system z3
<companion_cube>
but if you don't setup LD_LIBRARY_PATH (or sth like that) properly you'd be linking against your old 4.4.1
<kakadu>
Oh....
<companion_cube>
it takes a while to compile, right?
<kakadu>
I looked at opam install command and thought that it installs only the bingings
<kakadu>
bindings*
<companion_cube>
hmm
<kakadu>
you obviously do some install preparation in the build script
<companion_cube>
no, it builds everything
<kakadu>
It seems that opam really installs new z3
<companion_cube>
if you look in the opam dir, you should see a libz3.so
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<Drup>
kakadu: ugh
<Drup>
no, I didn't
<Drup>
given the fact that z3overlay was never formally release, I don't care about compatibility
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<hdon>
hi all :) is it possible for variant constructors to constrain their arguments beyond type? pseudo-code illustration: type pet = Dog(numLegs:int) when numLegs <= 4
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<thizanne>
hdon: no
<hdon>
oh. what options are available for me? wrap the constructor in a function?
<thizanne>
probably
<hdon>
mmm thanks thizanne
<mrvn>
make the constructor private with a function that checks for numLegs <= 4 and creates one.
<thizanne>
abstract the type away, provide functions that create values of this type, and make sure that your functions enforce your invariant
<hdon>
mmm good thinking. thanks both!
<mrvn>
type private pet = Dog of int has the benefit that anyone can read the number of legs but only your function can create a Dog.
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<mrvn>
type numLegs = One | Two | Three or Four is also an option
<hdon>
mrvn: mmm... good point mrvn. i do end up writing a lot of intOfNumLegs and numLegsOfInt sort of functions though :3
<mrvn>
yep.
<companion_cube>
you can also write a private record and use the field directly
<companion_cube>
type dog = private {legs: int}
<thizanne>
it's hard to tell what the actual right solution of your problem is without knowing more about your problem than needing to be able to cut off dogs' legs
<mrvn>
you might also have 1/2 eyes, 1/2/3 heads, bool thumbs / claws / hooves.
* hdon
nods
<mrvn>
If you have a bunch of those then a record or object is the way to go
<mrvn>
object if you have cats, dogs, sheep, ...
<mrvn>
Ocaml could really use a type t = SmallInt of 5; I've needed that often.
<companion_cube>
problem is decidability of type checking, as always
<mrvn>
companion_cube: should be just as difficult as type numLegs = One | Two | Three | Four.
<companion_cube>
you could do with runtime assertions inserted by the compiler, of course, but that's not a trivial decision to make
<mrvn>
fun x y -> SmallInt (x + y) would be a problem.
<companion_cube>
worse: fun x y -> SmallInt (x * y)
<thizanne>
sounds like a ppx could actually do this
<mrvn>
thizanne: doing that with runtime checks would be doable.
<thizanne>
or just translating [%smallint 5] to `Five
<mrvn>
thizanne: yep. That would limit everything to literals. No (x + y) possible.
<mrvn>
Often that's enough.
<thizanne>
and having a smallint + that either does runtime checking or does `Tree + `Four = `Five
<mrvn>
If you do the later then you might as well just have a constuctor function that checks the invariant.
<mrvn>
well, except for match legs with 0 | 1 | 2 | 3 -> "cripple" | 4 -> "healthy"
<mrvn>
Transforming to `Number would do range checking.
<mrvn>
thizanne: let me know when you have a [%smallint x] ppx :)
<thizanne>
I won't
<mrvn>
:(
<thizanne>
but that does seem like something you could have
<thizanne>
(or just a module)
<companion_cube>
or you can use ppx_deriving with `type t = One | Two | Three | Four` to at least have the int conversion
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