This post is related to issues addressed in
<https://github.com/nim-lang/RFCs/issues/380> and it attempts to solve them.
I'll quote some statements from there (copyrights not mentioned)
A remarkable observation:
> Instead of attaching syms to types, I think we should attach scopes (the
> declaration scope) to syms, so that symbols available at declaration will
> always be available at instantiation.
This is a description of a sym as a scope predicate, or, said otherwise, it has
a scope predicate. Predicates are available in Nim already, they are called
concepts. Scope concepts update the scope, they have an effect. This can be
expressed within different ways. I choose the star "*" symbol (a pragma could
do the same) The example is taken from the RFC as well and it looks like this:
# Module objs.nim
import hashes
type
Obj* = object
x*, y*: int
z*: string # to be ignored for equality
proc `==`*(a, b: Obj): bool =
a.x == b.x and a.y == b.y
proc hash*(a: Obj): Hash =
$!(hash(a.x) &! hash(a.y))
# Module bindtable.nim
import tables,hashes
export tables # potentially redundant
type
BTable[T,V] = concept
type U* = Table[T,V]
Self : U
proc `==`*(a, b: T): bool
proc hash*(a: T): Hash
#an example for a proc with the eigentype U
#proc getval*(t: U, k: int) : int
# main.nim
from objs import Obj
import bindtable
var t: BTable[Obj, int]
t[Obj(x: 3, y: 4, z: "debug")] = 34
echo t[Obj(x: 3, y: 4, z: "ignored")]
Run
