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ChrysaLisp Masterclass: Advanced Data-Driven Dispatch

Gemini 3.1 Pro quite likes the some construct used in the Canvas loader helpers !

One of the most powerful aspects of ChrysaLisp is its ability to combine the malleability and expressiveness of high-level Lisp with the raw, mechanical performance of C.

This masterclass explores a specific, highly optimized pattern for data-driven function dispatch. We will take a standard, bulky cond statement and transform it into a hyper-fast, compile-time-resolved iteration loop.

The Problem: The Bulky cond

Consider a typical file loader that checks file extensions to determine which decoding function to call. A standard approach uses a cond block:

(defun canvas-info (file)
	(or (if (defq stream (file-stream file))
			(cond
				((ends-with ".cpm" file) (pixmap-cpm-info stream))
				((ends-with ".flm" file) (pixmap-cpm-info stream))
				((ends-with ".tga" file) (TGA-info stream))
				((ends-with ".svg" file) (SVG-info stream))
				((ends-with ".cwb" file) (CWB-info stream))))
		(list -1 -1 -1)))

While readable, this is repetitive and requires evaluating a series of conditional branches at runtime. If we add 10 more image formats, the code becomes unwieldy.

The Solution: Concurrent some Iteration

Here is the fully optimized ChrysaLisp idiom to solve this:

(defun canvas-info (file)
	(or (if (defq stream (file-stream file))
			(some (# (if (ends-with %0 file) (%1 stream)))
				  '(".cpm" ".flm" ".tga" ".svg" ".cwb")
				  (const `'(,pixmap-cpm-info ,pixmap-cpm-info ,TGA-info ,SVG-info ,CWB-info))))
		(list -1 -1 -1)))

In just four lines, we have packed an immense amount of ChrysaLisp's core philosophy. Let's break down exactly why this works and why it is so fast.

1. The some Loop and Concurrent Iteration

In ChrysaLisp, some (and its imperative cousin some!) iterates over sequences and returns the first truthy value evaluated by its lambda.

Crucially, ChrysaLisp's iterators seamlessly support concurrent list iteration. By passing two lists to some, the lambda receives elements from both lists simultaneously. As soon as (ends-with %0 file) matches, it executes the loader (%1 stream), and some immediately halts and returns that result.

2. The # Anaphoric Lambda

Notice the use of # instead of lambda.

(# (if (ends-with %0 file) (%1 stream)))

This is the anaphoric lambda shortcut. It automatically binds incoming arguments to the globally interned symbols %0, %1, %2, etc.

  • %0 maps to the current element in the first list (the string ".cpm").

  • %1 maps to the current element in the second list (the function pointer).

Performance Note: Because %0 and %1 are static, globally interned symbols, they maintain a stable str_hashslot. This guarantees an O(1) cache hit during environment lookup at runtime, making the # macro significantly faster than a standard lambda with named parameters.

3. The const and Quasi-Quote Magic

This is where the true compile-time magic happens:

(const `'(,pixmap-cpm-info ,pixmap-cpm-info ,TGA-info ,SVG-info ,CWB-info))

Why do we use const and quasi-quoting instead of a simple (list ...) or a quoted literal '(... )?

  1. The not_a_function Trap:

    If we used (static-q (pixmap-cpm-info TGA-info ...)), ChrysaLisp's prebind pass (which optimizes ASTs) would see a list and assume it is a function call. It would attempt to bind pixmap-cpm-info as a function, but leave TGA-info and the rest as raw, unresolved symbols. At runtime, attempting to execute a symbol via (%1 stream) throws a not_a_function ! error.

  2. Compile-Time Resolution:

    To fix this, we need the compiler to evaluate the symbols into actual function pointers before the program runs.

    • const tells the compiler: "Evaluate the following expression right now, during compilation, and bake the result directly into the binary."

    • `'(,A ,B) uses quasi-quoting ` to create a template, and unquoting , to force the compiler to look up the function pointers for A and B in the current REPL environment.

    • The outer ' (quote) ensures that the resulting list of raw memory addresses/function objects is treated as literal data, not executable code.

The Result

When the ChrysaLisp compiler is finished with this block, there are no runtime symbol lookups, no eval overhead, and no deep conditional trees.

The binary simply contains a highly-optimized loop stepping through an array of strings and an array of raw function pointers, executing the correct one in O(1) time.

It is the readability and dynamism of Lisp, executing with the mechanical efficiency of a C jump table.

Using (const ...) to force compile-time evaluation, combined with the quasi-quote `'(,A ,B) to evaluate the symbols into function pointers and return a literal list, is a masterclass in ChrysaLisp macros. It completely bypasses the prebind AST logic by handing the compiler a fully resolved list of function objects, while keeping the scope perfectly tight (no need for a module-level defq). Zero runtime overhead, zero namespace pollution.