Modules

A module is a file. A file is a module. Everything else about Sure's module system follows from refusing to complicate that.

Importing

import geometry loads geometry.sure from the same directory as the importing file — once, no matter how many files import it. Its functions and structs are then reached through its name:

// geometry.sure
struct Point {
    x: Float,
    y: Float,
}

func origin() -> Point {
    return Point { x: 0.0, y: 0.0 }
}

// main.sure
import geometry

func main() -> Int {
    let p: geometry.Point = geometry.origin()
    print(p.x)
    return 0
}

Qualified names are not a politeness — they are the rule. An imported function cannot be called bare; there is no way to make origin() mean geometry.origin(), and therefore no way to wonder which file a name came from. What C solves with headers, include guards, forward declarations, and hope, Sure solves by not offering the problem.

Directories and names

import math.geometry loads math/geometry.sure. The module is still referenced by its last name — geometry.origin() — because the directory is where it lives, not what it is. When two modules would go by the same name, rename one for your file with as:

import math.geometry
import screen.geometry as sgeo

func convert(p: geometry.Point) -> sgeo.Point {
    // both geometries, no ambiguity, no guessing
    return sgeo.from_world(p)
}

Without the as, that pair of imports is a compile error naming both files and telling you to pick. Sure will not guess which geometry you meant, because guessing is how C linkers work.

Rules with reasons

• Import cycles are errors. module cycle: a imports b imports a. A dependency loop is a design smell wearing a build system as a disguise.
• Modules load once. Two files importing base share one base — its structs are the same types, its state would be the same state, if Sure had global state, which it doesn't.
• A module's own imports are its own. If geometry imports trig, you do not get trig by importing geometry. Import what you use, where you use it.
• Module names are reserved. You cannot declare a variable named geometry in a file that imports it, and a module is not a value — let g = geometry is refused on principle.

Errors know their file

When something is wrong inside an imported module, the diagnostic says where:

🫤 math/geometry.sure line 8: not sure about this
    'origin' returns Point, not Float