Control Flow in build.fol

build.fol is a real FOL program. It supports when, loop, and user-defined helper routines.

when

when conditionally executes build operations based on a boolean expression.

The canonical form for a conditional block with no case arms uses a double-brace default body:

when(optimize == "release-fast") {
    {
        var strip_step = graph.step("strip");
        var packed = graph.add_system_tool({
            tool   = "strip",
            output = "gen/app.stripped",
        });
        strip_step.attach(packed);
    }
};

With explicit case arms:

when(target) {
    case("x86_64-linux-gnu") {
        graph.step("asan", "Enable address sanitizer");
    }
    case("aarch64-linux-gnu") {
        graph.step("tsan", "Enable thread sanitizer");
    }
    * {
        graph.step("default-check");
    }
};

The inner double-brace { { ... } } is required for the default arm when no case clauses are present. This is how the FOL parser distinguishes the default body from raw statements.

Conditional Expressions

Any boolean or option value comparison is valid:

when(optimize == "release-fast") { { ... } };
when(strip == true) { { ... } };
when(target == "x86_64-linux-gnu") { { ... } };

The comparison is resolved at build evaluation time using the values passed via -D flags. If no value was provided, the declared default is used.

loop

loop iterates over a list and runs the body for each element.

loop(name in {"core", "io", "utils"}) {
    graph.add_static_lib({ name = name, root = name });
};

The loop variable (name) is bound in scope for each iteration. It can be used anywhere inside the body as a string value.

Loop over a list with multiple fields:

loop(name in {"core", "io"}) {
    var lib = graph.add_static_lib({ name = name, root = name });
    graph.install(lib);
};

The iterable is a container literal { elem1, elem2, ... }. Currently only string lists are supported as loop iterables in build.fol.

Helper Routines

build.fol can define helper fun[] and pro[] routines. They are visible only within the file — they are not exported to the package.

Helper Function

fun[] make_lib(name: str, root: str): Artifact = {
    return .build().graph().add_static_lib({ name = name, root = root });
}

pro[] build(): non = {
    var build = .build();
    build.meta({ name = "app", version = "0.1.0" });
    var graph = build.graph();
    var core = make_lib("core", "src/core/lib.fol");
    var io   = make_lib("io",   "src/io/lib.fol");
    var app  = graph.add_exe({ name = "app", root = "src/main.fol" });
    app.link(core);
    app.link(io);
    graph.install(app);
    graph.add_run(app);
}

The helper make_lib accesses the graph through .build().graph(). The graph handle is not a public type name and is not passed as a user-declared parameter.

Helpers Calling Helpers

Helpers can call other helpers:

fun[] lib_root(name: str): str = {
    return "src/" + name + "/lib.fol";
}

fun[] add_lib(name: str): Artifact = {
    return .build().graph().add_static_lib({ name = name, root = lib_root(name) });
}

pro[] build(): non = {
    var build = .build();
    build.meta({ name = "app", version = "0.1.0" });
    var graph = build.graph();
    var core = add_lib("core");
    var app  = graph.add_exe({ name = "app", root = "src/main.fol" });
    app.link(core);
    graph.install(app);
}

Combined Example

fun[] make_lib(name: str): Artifact = {
    return .build().graph().add_static_lib({ name = name, root = name });
}

pro[] build(): non = {
    var build = .build();
    build.meta({ name = "app", version = "0.1.0" });
    var graph = build.graph();
    var target   = graph.standard_target();
    var optimize = graph.standard_optimize();
    var strip    = graph.option({ name = "strip", kind = "bool", default = false });

    loop(name in {"core", "io", "net"}) {
        make_lib(name);
    };

    var app = graph.add_exe({
        name     = "app",
        root     = "src/main.fol",
        target   = target,
        optimize = optimize,
    });
    graph.install(app);
    graph.add_run(app);

    when(strip == true) {
        {
            var strip_step = graph.step("strip");
            var packed = graph.add_system_tool({
                tool   = "strip",
                output = "gen/app.stripped",
            });
            strip_step.attach(packed);
        }
    };
}