"opening" datatypes

[YawarRaza7349]

This is a proposal that could actually be added to Successor ML implementations (SML/NJ and MLton).

I was looking the MLton codebase and noticed code like this popping up everywhere.

datatype z = datatype Control.Target.arch
datatype z = datatype Control.Target.os
datatype z = datatype Control.Format.t
datatype z = datatype Control.codegen

I had no idea what the point of creating an "alias" and immediately shadowing it would be. I had to look at several examples to realize the purpose of this: it's to make the constructors of each datatype available without qualification.

It would be much clearer if, instead of a weird idiom, this could be expressed directly as:

open datatype Control.Target.arch
open datatype Control.Target.os
open datatype Control.Format.t
open datatype Control.codegen

Or even:

open datatype Control.Target.arch Control.Target.os
  Control.Format.t Control.codegen

Since datatype is a keyword, it can't be the name of a module, so there's no ambiguity with open for modules.

[Hibou57]

It's a wise idea, indeed. On an other hand, if constructors of a type are used, the corresponding type name will probably be too, unless one only rely on type inference. Personally, I prefer explicit type annotation for comprehension, so I never noticed that point. Still a good idea.