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Lisp Project of the Day

april

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aprillanguage

"April" is a subset of the APL (A Programming Language).

Why does APL can be interesting?

  • it was developed in the 1960s.
  • it is able to work with multidimensional arrays is a very concise manner.
  • it uses special graphic symbols to represent most functions and operators.
  • it is not a "1 April joke".

You can replace hundreds of lines of number-crunching code with a single line of APL.

Personally, I'm not familiar with this language and APL code seems very cryptic to me. But if you are working with numbers and algorithms, probably APL worth thing to look at.

"April" contains an embedded test suite and a demo. Here is a piece of its demo output:

POFTHEDAY> (april:april (demo))

...  

∇ Monadic inline function.
  _ {⍵+3} 3 4 5
    6 7 8
    
∇ Dyadic inline function.
  _ 1 2 3 {⍺×⍵+3} 3 4 5
    6 14 24
    
∇ Vector of input variables and discrete values processed within a function.
  _ fn←{3+⍵} ⋄ {fn 8 ⍵} 9
    11 12

...

Here is how you can eval the same programs from the Lisp REPL:

POFTHEDAY> (april:april "{⍵+3} 3 4 5")
#(6 7 8)

POFTHEDAY> (april:april "1 2 3 {⍺×⍵+3} 3 4 5")
#(6 14 24)

POFTHEDAY> (april:april "fn←{3+⍵} ⋄ {fn 8 ⍵} 9")
#(11 12)

The cool thing is that "april:april" is a macro. It is expanded into the Lisp code which can be compiled into the native code and executed very fast:

POFTHEDAY> (macroexpand-1 '(april:april "1 2 3 {⍺×⍵+3} 3 4 5"))
  
(PROGN
  NIL
  (LET* ((APRIL::OUTPUT-STREAM *STANDARD-OUTPUT*)
         (APRIL::INDEX-ORIGIN 1)
         (APRIL::PRINT-PRECISION 10))
    (DECLARE
     (IGNORABLE APRIL::OUTPUT-STREAM APRIL::INDEX-ORIGIN
                APRIL::PRINT-PRECISION))
    (APRIL::APL-OUTPUT
     (APRIL::APL-CALL :FN
                      (LAMBDA (APRIL::⍵ &OPTIONAL APRIL::⍺)
                        (DECLARE (IGNORABLE APRIL::⍵ APRIL::⍺))
                        (APRIL::APL-CALL × (APRIL::SCALAR-FUNCTION *)
                                         (APRIL::APL-CALL +
                                                          (APRIL::SCALAR-FUNCTION
                                                           +)
                                                          3 APRIL::⍵)
                                         APRIL::⍺))
                      (APRIL::AVECTOR 3 4 5) (APRIL::AVECTOR 1 2 3))
     :PRINT-PRECISION APRIL::PRINT-PRECISION)))

It is worth to read April's documentation. It is full of examples.


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