extend

.extend name:{String} \
       where:{Lambda? = null} \
        body:{Lambda}
-> Void

Defines a custom function that extends an existing one by name, wrapping its output.

Within the body, the original function is exposed as .super: calling it invokes the original with the arguments the wrapper was called with. The body's explicit parameters, if any, must match the original function's parameters by name and let the wrapper read those same arguments. All wrapper parameters are optional, regardless of whether the original parameters are.

.function {greet}
    name:
    Hello, .name

.extend {greet}
    .super::uppercase

.greet {World}

Output: HELLO, WORLD

The wrapper can reference the original parameters by name to alter the behavior conditionally:

.extend {greet}
    name:
    .if {.name::equals {World}}
        .super::uppercase
    .ifnot {.name::equals {World}}
        .super

Because .super is the original function itself, it accepts the same arguments. Anything you pass overrides the corresponding argument the wrapper was called with, while anything you leave out falls through unchanged:

.extend {greet}
    name:
    .super name:{.name::uppercase}

.greet {John}

Output: Hello, JOHN

If where is defined, it is checked against every function call. If the condition is not met, the call falls back to the original function definition. The following snippets have identical behavior:

•   .extend {greet} where:{name: .name::equals {World}}
        name:
        .super::uppercase

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•   .extend {greet}
        name:
        .if {.name::equals {World}}
            .super::uppercase
        .ifnot {.name::equals {World}}
            .super

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Function extension is the backbone of element styling, when extending primitive functions such as heading.

Parameters

Throws