Is there a good way to check if a variable is a variant with a particular tag without doing a match? I think the answer is no. My motivation is this: I want to write a small function mustbe a b that confirms that the variant b has the tag given in a, as in mustbe Float x or some such. However, I …

[image] yawaramin: Are you using a parser combinator library or Menhir currently? Well, to explain, I’m using Menhir for my main project, but I wanted to knock off a small task with a primitive recursive descent + parser combinator hack of my own. I wanted a family of functions to recognize …

BTW, just as a pure aside (it has no real impact on the current discussion at all), why can’t functions be compared for equality? Clearly you can’t tell that two distinct functions that do the same thing are equal, but you can tell if two functions are identical, can’t you?

[image] perry: I’m using Menhir for my main project Oh, and as another aside: I’m finding the modern “easy” way to do error messages for Menhir not very easy at all. The explanations in the manual may be at fault. :frowning:

You can compare functions for being identical (==). You just can’t compare for equality (=). But you might not get the answer you would want due to inlining and such. # sin == sin;; - : bool = false # let f x = x + 4;; val f : int -> int = <fun> # f == f;; - : bool = true Since functions are immut…

I got a little bored, so I decided to try my hand at writing a ppx rewriter for this. I haven’t bothered packaging it yet, and feedback is quite welcome, but it works like [%constr_test Some __] (Some 1). [image] leviroth/ppx_constr_test Test if a variant matches a given constructor. Cont…

As I understand it, it’s because a function is more than its definition, it also contains its closure, so what you would really need is a way of comparing closures.

Ah, this just made me remember something possibly from Prof. Dan Grossman’s Programming Languages course. In a closure, a lot of type information is actually hidden. E.g.: let message = "Hi" let f test = if test then print_endline message Here f has type bool -> unit, but in reality its closure a…

But f in this case is probably represented as some address in memory, and if I compare that to the address in memory of any other function, it will be different, and if the other function has the same address, even if I don’t know its type details, I can be pretty sure it is the same function. Or am…

[image] Levi_Roth: I got a little bored, so I decided to try my hand at writing a ppx rewriter for this. Very cool! I have to say I’m still uncomfortable with writing ppx extensions (though using them is quite straightforward!) — I wish the documentation was a lot better.

The definition of (==) makes almost no guarantee when comparing immutable things. The only guarantee is that if the things are physically equal (==) they will also be equal (=). Since functions can’t be compared for equality there’s no way to even verify this. If you’re very knowledgeable about the …

Checking a variant's tag without a match?

Learning

Kakadu January 5, 2018, 12:59pm 32

I think that we should add exactly this to the documentation of Obj module

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Checking a variant's tag without a match?

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