The meta Phase

When it comes to actual code execution, there is no point to a definition without a body. Thus, in order to eagerly know what definitions might be executed at compile time and so need to be available including their bodies (in some executable shape), any definition used as an entry point to compile-time execution has to be tagged with the new meta modifier. This is automatically done in built-in metaprogramming syntax such as syntax, macro, and elab but may need to be done explicitly when manually applying metaprogramming attributes such as @[app_delab].

A meta definition may access (and thus invoke) any meta or non-meta definition of the current module. For accessing imported definitions, the definition must either have been marked as meta when it was declared or the import must be marked as such (meta import when the accessing definition is in the private scope and public meta import otherwise).

module

meta import Std.Data.HashMap

local elab "my_elab" : command => do
  let m : Std.HashMap := {}
  ...