Addons should be able to inject codegen at predefined location. Describe and discuss how to obtain this

(see design document)

Codegen Extension

How? Two approaches discussed in the following

Placeholders

• Addon is given an instantiated template (e.g. CompilationUnit)
• Addon replaces arbitrary placeholders

Example

interface Addon {
    void replacePlaceHolders(CompilationUnit u);
}
class MyAddon implements Addon {
    void replacePlaceHolders(CompilationUnit u) {
        u.findFirst(MethodCallExpr.class,
                    m -> m.getNameAsString().equals("$someMethod$"))
                .ifPresent(u.replace(...));
    }
}

PROS: 

• Simplest approach
• It is the same pattern used in “main” codegen
• Potentially at some point we may align “main” codegen and treat everything as addons
• Addons may do anything

CONTRAS:

• Addons may do anything
• Harder to limit/predict what it is doing/where it is breaking

Plug-In Mechanism

• Addons may only hook into specific extension points; e.g. implement an interface
• The system may replace known placeholders with addon-supplied AST nodes, by invoking the interface methods

Example

interface Addon {
    Expression augmentSomeMethod(MethodCallExpr m);
}
class MyAddon implements Addon {
    Expression augmentSomeMethod(MethodCallExpr m){
        // wrap m, and return the new expression
    }
}

PROS: 

• Addons may only hook into specific extension points => easier to control/more predictable

CONTRAS:

• Addons are second-class citizens, as opposed to “main” codegen. Not necessarily a contra.

The exercise may even help to find out that also “main” codegen only hooks into very specific points => define interfaces for all components__

• It may be uglier to scale if the number of extension points grows; extension points may become ad-hoc, e.g. worst case, we may have many extension points, but each addon only replace 1 distinct placeholder (i.e. no overlaps)

However we we may use “many interfaces” instead of 1 interface with many methods_

e.g.:

interface SomeMethodAugmenter {
    Expression augment(MethodCallExpr m);
}
interface SomeOtherMethodAugmenter {
    Expression augment(MethodCallExpr m);
}