Programs represented as lists

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Expressions, Types, and Functions - slide 25 : 46

Programs represented as lists

It is a unique property of Lisp that programs are represented as data, using the main data structure of the language: the list

A sample Scheme program from the LAML library:

(define (as-number x)
  (cond ((string? x) (string->number x))
        ((number? x) x)
        ((char? x) (char->integer x))
        ((boolean? x) (if x 1 0))  ; false -> 0, true -> 1
        (else
         (error
          (string-append "Cannot convert to number "
                         (as-string x))))

In Scheme it is not intended that the program source should be introspected by the running program

But in other Lisp systems there is easy access to self reflection

R5RS: Vectors