A local-var can't be stored in a type

[anfelor]

For example, this definition of a graph:

private struct graph<h> {
    nodes_ : local-var<h, vector<node>>
    num_edges_ : int
}

yields

graph.kk(49, 5): error: types do not match
  context      :     nodes_
  term         :     nodes_
  inferred type: vector<node>
  expected type: local-var<$h,vector<node>>
  because      : function returns values of different types

Is this expected behavior?

EDIT: One can use ref instead of local-var which makes updates of the vector a bit more cumbersome but works as expected.

[daanx]

Hi, good question/issue. I will try to document this more -- the local-var is an internal type used by var declarations; these are limited as their life-time is always limited to their lexical scope (checked through the h parameter). However, by limiting them we can give them the exact semantics of a state effect and create (in the future) more efficient implementations.

The ref<h,a> is the general type of references that are first-class and can have life times that exceed the lexical scope. An example of references is in the samples/basic/garsia-wachs example.

Hope this helps

[anfelor]

Thank you, yes, ref seems to be the better option. I originally used local-var because ([]) for vectors uses it.. maybe the function should also be provided for ref?

[daanx]

Ah, the vector assignment right; Yes, I think we need this for ref as well -- it needs some re-thinking now that Perceus is implemented; when vectors are unique (-ly owned) they can be updated in-place. That makes the need for ref's to vectors less needed perhaps? In particular updating an element in the vector that the ref points too. Not sure :-)