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Comment by ncmncm | Hacker News Reader

Without watching the video, can we guess correctly that "for Common Lisp" means "in Common Lisp"? Or is it really meant for use within Common Lisp programs?

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April is usable within Common Lisp. When writing a CL program, you can invoke APL code on arrays. For example:

(april-c "{⍴∪,(3⍴2*8)⊥3 1 2⍉⍵}" (opticl:read-png-file "/path/to/image.png"))

This snippet uses the opticl library to load a PNG file, and then uses April to count the number of unique colors in the image. Consider the amount of code this would take in CL.

It would be cool if it was implemented as a reader macro instead of just passing a string to a function.

Do it yourself! (Implementation loosely based on phantomics' comment below.)

    (defun april-reader (stream char)
      (declare (ignore char))
      `(april ,(read stream t nil t)))
    
    (set-macro-character #\⎕ #'april-reader)

You're probably gonna want something more complicated than that. Glancing at the github page it looks like at least the following need to be dealt with:

  (april "apl code") ; and april-f
  (april (with (stuff)) "apl code") ; and april-f
  (april-c "apl code" (input))
  (april-c (with (stuff)) "apl code" (input))

Yes, the above is a trivial implementation; handling multi-form input would need to be much more complex. Thankfully, for simple one-liners, you'll likely want either April literals (implemented above) or April anonymous functions, both of which won't take additional arguments; you should be able to implement both as simple reader macros.

Wouldn't be too hard to do, someone in the video suggested a #⎕ reader macro followed by APL. Like take '#⎕string' then expand that to '(april "string")'.

But April has many ways of passing options and external data for the compiled code to operate upon, and implementing a reader macro system that would support all those parameters would be complicated and require developers to learn a bunch of new syntax in order to use April with reader macros.

Another option is to load APL source from a file.

(april-load #P"/path/to/code.apl")

Then you can have files of pure APL code with no Lisp hanging around.

> Consider the amount of code this would take in CL.

I don't really read APL - but isn't this just a map/reduce over the pixels? - maybe (length (remove-duplicates 'pixel-data)) or some such?

The pixels aren't singular values, the image is a height×width×3 array of 8-bit integers. The third dimension is the three RGB values for each pixel. The bulk of the string, the "(3⍴2*8)⊥3 1 2⍉⍵" part, converts the array into a height×width matrix of 24-bit integers representing the colors. Once you have a matrix M, "⍴∪,M" is all the code needed to count the unique values.

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