6 ms·

Namespacing Variants in ML

59 points by halst 11y ago

mercurial 11y ago
Or you can use polymorphic variants in OCaml: type color = `Red | `Green | `Blue | `Black | `Brown type hair_color = `Black | `Brown | `Blond
- jallmann 11y ago
  In OCaml 4.01, the compiler is smart enough to distinguish between constructors with the same name but a different type -- so those variants wouldn't even need to be polymorphic anymore. This also partially mitigates the need for 'namespacing' your variants.
  - mercurial 11y ago
    Oh, that's pretty nice. OCaml really gets namespacing problems out your way.
gabelevi 11y ago
I'm also a fan of namespacing variants, and I used it extensively on an OCaml representation of the SpiderMonkey AST. (Coincidentally, up until writing our own parser we were using pfff's JavaScript parser! Go pfff!). Anyway, here are some other useful things I found in OCaml along the way: Without namespacing, you might be using mutually recursive type definitions like type statement = ExpressionStatement of expression and expression = FunctionExpression of statement list to do this with modules you need to use recursive modules, like module rec Statement : sig type t = Expression of Expression.t end = struct type t = Expression of Expression.t end and Expression : sig type t = Function of Statement.t list end = struct type t = Function of Statement.t list end One of the sucky things about recursive modules is that you cannot omit the signatures, however if your modules only include types then there is a shortcut that looks like module rec Statement : sig type t = Expression of Expression.t end = Statement and Expression : sig type t = Function of Statement.t list end = Expression Links: Big real world example: https://github.com/facebook/flow/blob/3a5b4040b7d2a648f97a06c93d9dc352c3e2b008/src/parser/spider_monkey_ast.ml#L85 https://github.com/facebook/flow/blob/3a5b4040b7d2a648f97a06... Recursive module docs: http://caml.inria.fr/pub/docs/manual-ocaml-400/manual021.html#toc75 http://caml.inria.fr/pub/docs/manual-ocaml-400/manual021.htm...
padator 11y ago
One thing to consider though is the grepability of a codebase. By relying more on nested modules, module aliases, opens, local opens, then many constructors have more than one way to be written and it makes it harder for grep to find what you look for. Of course people should not use grep and instead rely on IDE or tools that understand deeply the programming language being edited ... which makes the problem I mentioned go away. But still it's nice to have the possibility to use simple tools like grep and that they would simply work.

59 points by halst 11y ago

jallmann 11y ago
In OCaml 4.01, the compiler is smart enough to distinguish between constructors with the same name but a different type -- so those variants wouldn't even need to be polymorphic anymore. This also partially mitigates the need for 'namespacing' your variants.
- mercurial 11y ago
  Oh, that's pretty nice. OCaml really gets namespacing problems out your way.

gabelevi 11y ago

I'm also a fan of namespacing variants, and I used it extensively on an OCaml representation of the SpiderMonkey AST. (Coincidentally, up until writing our own parser we were using pfff's JavaScript parser! Go pfff!). Anyway, here are some other useful things I found in OCaml along the way: Without namespacing, you might be using mutually recursive type definitions like type statement = ExpressionStatement of expression and expression = FunctionExpression of statement list to do this with modules you need to use recursive modules, like module rec Statement : sig type t = Expression of Expression.t end = struct type t = Expression of Expression.t end and Expression : sig type t = Function of Statement.t list end = struct type t = Function of Statement.t list end One of the sucky things about recursive modules is that you cannot omit the signatures, however if your modules only include types then there is a shortcut that looks like module rec Statement : sig type t = Expression of Expression.t end = Statement and Expression : sig type t = Function of Statement.t list end = Expression Links: Big real world example: https://github.com/facebook/flow/blob/3a5b4040b7d2a648f97a06c93d9dc352c3e2b008/src/parser/spider_monkey_ast.ml#L85 https://github.com/facebook/flow/blob/3a5b4040b7d2a648f97a06... Recursive module docs: http://caml.inria.fr/pub/docs/manual-ocaml-400/manual021.html#toc75 http://caml.inria.fr/pub/docs/manual-ocaml-400/manual021.htm...

padator 11y ago

One thing to consider though is the grepability of a codebase. By relying more on nested modules, module aliases, opens, local opens, then many constructors have more than one way to be written and it makes it harder for grep to find what you look for. Of course people should not use grep and instead rely on IDE or tools that understand deeply the programming language being edited ... which makes the problem I mentioned go away. But still it's nice to have the possibility to use simple tools like grep and that they would simply work.