Boot (more precisely newer dialect called Shoe) is used internally in FriCAS for some of the interpreter and compiler code. Here is a detailed description of the Boot Language

This is a simple example of Boot programming.

Define a function.

pairBoot(a,b) ==
  a>b => [-b,-a]
  [a,b]

9211484592716052519-25px001.clisp PRODUCED
; compiling file "/var/aw/var/LatexWiki/9211484592716052519-25px001.clisp" (written 17 JUL 2019 05:17:24 PM):

; /var/aw/var/LatexWiki/9211484592716052519-25px001.fasl written
; compilation finished in 0:00:00.003

Now call it.

pairBoot(1,4)$Lisp

(1)

map(integer,destruct(pairBoot(3,2)$Lisp))

(2)

That's my first Boot function!

You can now write for example:

foobar(x,y) == x + y

7491012761112044243-25px003.clisp PRODUCED
; compiling file "/var/aw/var/LatexWiki/7491012761112044243-25px003.clisp" (written 17 JUL 2019 05:17:24 PM):

; /var/aw/var/LatexWiki/7491012761112044243-25px003.fasl written
; compilation finished in 0:00:00.001

And call it with

integer(foobar(2,3)$Lisp)

(3)

In FriCAS (from revision 1049 on) the simplest way to compile and load boot code is to write it into a file and then call something like:

  )read FILE.boot

To debug Boot code add diagnostic printouts using SAY, like

print_bar(x) ==
   SAY(["print_bar", x])

6147864484223767607-25px005.clisp PRODUCED
; compiling file "/var/aw/var/LatexWiki/6147864484223767607-25px005.clisp" (written 17 JUL 2019 05:17:24 PM):

; /var/aw/var/LatexWiki/6147864484223767607-25px005.fasl written
; compilation finished in 0:00:00.001

)boot print_bar(123)

(EVAL-WHEN (EVAL LOAD) (PROG () (RETURN (|print_bar| 123))))
(print_bar 123)
Value = NIL

Note: Unlike other printing functions SAY prints to terminal even if normal output is redirected to a file. So it is useful for debugging for example Spad compiler, where normal output goes to file.

One can also add breakpoints using BREAK(). When executed BREAK() passes control to Lisp debugger and one can for example find out which code called given place. BREAK() is used in FriCAS sources to indicate conditions that should be always true, but we want visible report if they fail.