[Grace-core] Types of blocks
Kim Bruce
kim at cs.pomona.edu
Mon Aug 22 18:11:50 PDT 2011
On Aug 22, 2011, at 12:08 AM, Michael Homer wrote:
> Hi,
> I've begun working on typechecking in my implementation, and have a
> (primarily nominal) static checker working almost everywhere. There
> are no generics so far, but I've begun thinking about how the typing
> of blocks will work. To begin with, just how do they work? I have been
> presuming they are genericised in the parameters and return type
> somehow, but exactly how that should look is a little hazier,
> particularly if a) blocks can be variadic or b) there is a meaningful
> common supertype of blocks.
Each block has a signature that is our representation of function types. If it has parameters a:A, b: B and returns a result of type C then
the type is Function(A,B,C) (informally), which translates to an objecttype with an apply method with parameters of type A, B and return type C.
>
> As well, there are a couple of points about where those types come
> from: what exactly is inferred? It seems like the most obvious result
> is the disjunction of all possible return types, but there are other
> sensible answers too. What about a block containing a "return"
> statement (which hence "returns" nothing at all)?
In general you would take the sup of the return types (which happily exists in a structurally typed language). The return type of a block is the type of the last statement in the block (treating a stand-alone expression as a statement).
Most commands will have type Unit. E.g., the statement x:= 12 will have type Unit, not Number. Similarly, a bare return statement will return a value of type Unit. Our interpreter doesn't actually have return statements.
>
> Can blocks take generic parameters? By that I mean code like this:
> { a : T -> Proxy.new(a) }
> where "T" is a fresh type identifier. Is that permissible?
I would say no to that. However, I would be tempted to allow something like
[T]{a:T -> ...}
though you'd probably be more likely to actually name it:
def f[T] = {a:T -> ...}
At any rate, I would worry much about these -- ignore them for now.
> How is that
> typed, and where does inference go through them? Especially, how does
> the "apply" method turn out in this case?
There would be no type inference. Do you have a compelling example as to why we should include this?
>
> I am also interested in how generics and reified types will work more
> generally, but as I said I haven't started implementing them at all
> yet.
I think the generics should be relatively straightforward, and we discovered (much to our surprise) that we didn't need to reify the type parameters because they just got substituted in structurally.
>
> If anybody wants to look at what's there, my typechecker does run in
> the JavaScript version [1], and currently supports the types Number,
> String, and Boolean, along with any user types defined with "type X =
> { ... }", and union types with A | B. Values can move freely in and
> out of type Dynamic, with no casting and currently no runtime checking
> either. It checks the existence of methods and that argument and
> parameter types match, along with assignments, but does only very
> limited structural matching. There are test cases at the bottom
> showing how it behaves.
> -Michael
>
> [1] http://ecs.vuw.ac.nz/~mwh/minigrace/js/
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