ACNews #70 02 мая 2018

Listh language project - Forth is implemented much simpler than Lisp, but it has big drawbacks.

                     Listh language project
                        by Alone Coder

Lisp and Forth allow you to quickly deploy the programming
environment on a new machine or under a new OS. To do this, the
language itself is made very simple, but flexible.

Forth is implemented much simpler than Lisp, but it has big
drawbacks:
- function parameters are in the same stack as the result of the
function, and are inaccessible by name - it is not convenient to
read them;
- if some function is written with an error in the depth of the
stack, it will crash the whole program;
- complex mathematical operations are not readable, it is
impossible to write parentheses;
- the system does not provide dynamic lists, as well as general
dynamic memory;
- apparently, you can not pass a pointer to a function or a
closure.

Lisp also implements the editing environment on its own, and in
principle, it can always be changed by a user. And it can be
done right while the program is running.

I decided to eliminate Forth's flaws, while not implementing the
most intricate things in Lisp - contexts, garbage collection,
and parameter interpretation when calling functions. And also
avoiding the Lispian typing of list cells - not a true type
system, it also hinders the effective storage and execution of
the program. And as we remember, one of the advantages of Lisp
is that it can edit itself. The language is meant to be executed
directly from the source text.

My idea was that the basic data structure is the list, and the
parameters and result of the functions are passed to the
(pseudo-)stack, that is, when you call a function, you do not
need to interpret them. But at the same time:
- functions have the ability to read the terms of the source
text, that is, to receive parameters written on the right;
- you can provide for storing the depth of the stack when
calling, so that functions return the old depth by default (but
I have not done so yet);
- the type of a nested list is determined by its first element.
Terms are also lists, because they have symbolic names that must
be stored somewhere for the possibility of showing. In order not
to interpret this type in the first element, we simply write the
address of the handler of this type: variable, constant,
function... (For the built-in functions, there will be specific
handlers.) The second element of the list contains useful data
(the contents of the variable, the address of the function...),
and then - the name, character by chaaracter.

Expressions in this language will look something like this:
(a + (x))
(a + 0) //operations of type "+" read and evaluate their right
operand
((x) + a)
(f (x) + a)
(a + f (x))
(fquote (x y z)) //this function does not calculate its operand
(a b f2) //this function reads operands from the stack

Below is the code of the interpreter and a couple of built-in
function handlers (in fact, there should be much more). The base
function is called count. Pseudo-stack grows upward, towards the
real stack.

        macro PSEUDOPUSHHL
        ld a,l
        ld (bc),a
        inc c
        ld a,h
        ld (bc),a
        inc bc
        endm

        macro PSEUDOPUSHDE
        ld a,e
        ld (bc),a
        inc c
        ld a,d
        ld (bc),a
        inc bc
        endm

        macro PSEUDOPOPHL
        dec bc
        ld a,(bc)
        ld h,a
        dec c
        ld a,(bc)
        ld l,a
        endm

        macro PSEUDOPOPDE
        dec bc
        ld a,(bc)
        ld d,a
        dec c
        ld a,(bc)
        ld e,a
        endm

        macro GETNEXT
;read the data in hl
;read pnext in de
        ex de,hl
        ld e,(hl)
        inc l
        ld d,(hl) ;data
        inc l
        ld a,(hl)
        inc l
        ld h,(hl)
        ld l,a ;pnext
        ex de,hl
        endm

count:
;list interpreter
;calls the function on the pointer in de, it goes again to count
;or to the endcount
        GETNEXT ;read pfunc in hl, pnext in de
        ld a,(hl)
        inc l
        ld h,(hl)
        ld l,a ;*pfunc (in the function description list,
               ;the first element is the address for the call)
        jp (hl) ;when an asm function is called, the stack does
                ;not move

PushValue:
;asmfunction at the beginning of the variable/constant/list
;return value from the current list is in (de)
        ex de,hl
        ld e,(hl)
        inc l
        ld d,(hl) ;value
        PSEUDOPUSHDE
endcount:
;Called as the last item in the list
        ret ;endcount

Lisp:
;The asm function at the beginning of function descriptor list
;read value in the current list - lies in (de), this is the
;address of the list to be executed
        ex de,hl
        ld e,(hl)
        inc l
        ld d,(hl) ;value
        jp count ;calculation of the list where the function is
                 ;described, exit to the endcount

plus:
;called from the Lisp, so it does not contain ret
;left operand is in pseeudo-stack
;reads the function / variable / constant / list by pointer (and
;moves the pointer), calculates it, puts the result in
;pseudo-stack, adds 2 numbers to the pseudo-stack
        GETNEXT ;read plist in hl, pnext in de
         push de ;current pointer
        ex de,hl ;pointer = pdata
;de = pointer to a function / variable / constant / list, i.e.
;in fact, always to the list
;if it is a function / variable / constant, just call ((de))
;(exits to endcount)
;if it is a list, call count (with de moving and exit to
;endcount in the end of the list)
        call count ;execute the first term of the list and
                   ;continue until endcount
        PSEUDOPOPDE ;de is now right operand
        PSEUDOPOPHL ;hl is now left operand
        add hl,de
         pop de ;current pointer
        PSEUDOPUSHHL
        jp count

_readnext:
;called from the Lisp, so it does not contain ret
;reads term address from the pointer, pushes it in pseudo-stack
;(without evaluation), moves the pointer
        GETNEXT ;read term in hl, pnext in de
        PSEUDOPUSHHL
        jp count

_eval:
;called from the Lisp, so it does not contain ret
;reads term address from pseudo-stack, evaluated it, pushes the
;result in pseudo-stack
         push de ;current pointer
        PSEUDOPOPDE ;de is now term address
;de = pointer to a function / variable / constant / list, i.e.
;in fact, always to the list
;if it is a function / variable / constant, just call ((de))
;(exits to endcount)
;if it is a list, call count (with de moving and exit to
;endcount in the end of the list)
        call count ;execute the first term of the list and
                   ;continue until endcount
         pop de ;current pointer
        jp count

_POPVAR:
;called from the Lisp, so it does not contain ret
        GETNEXT ;read pvar in hl, pnext in de
        inc l
        inc l
        ld a,(hl)
        inc l
        ld h,(hl)
        ld l,a    ;pvar.next (in the descriptor list of the
;variable the first element is call address, second is value)
         push de
        PSEUDOPOPDE
        ld (hl),e
        inc l
        ld (hl),d
         pop de
        jp count

_DUP:
;called from the Lisp, so it does not contain ret
        PSEUDOPOPHL
        PSEUDOPOPHL
        PSEUDOPUSHHL
        jp count

_SWAP:
;called from the Lisp, so it does not contain ret
         push de
        PSEUDOPOPDE
        PSEUDOPOPHL
        PSEUDOPUSHDE
         pop de
        PSEUDOPUSHHL
        jp count

        ds (-$)&3 ;ALIGN 4
readnext:
        dw _readnext,$+2,0,$+2
        dw 'r',$+2,'e',$+2,'a',$+2,'d',$+2
        dw 'n',$+2,'e',$+2,'x',$+2,'t',NIL
eval:
        dw _eval,$+2,0,$+2
        dw 'e',$+2,'v',$+2,'a',$+2,'l',NIL
POPVAR:
        dw _POPVAR,$+2,0,$+2
        dw 'P',$+2,'O',$+2,'P',NIL
DUP:
        dw _DUP,$+2,0,$+2
        dw 'D',$+2,'U',$+2,'P',NIL
SWAP:
        dw _SWAP,$+2,0,$+2
        dw 'S',$+2,'W',$+2,'A',$+2,'P',NIL
        ...
freemem:
        dup ($-endmem)/4
        dw 0,$+2
        edup

How we will do cycles and conditions:
(1 2 ifeq (...) (..else ..))) - how to transfer the calculated
parameters to the body? through the stack and global variables?
loop (...) - how to transfer the calculated parameters to its
body? How to get out? (you can provide a loop-exit hack, but the
stack will not be able to store the exit address, because exit
can be parenthesised)
use only global variables?
and provide for them commands "push" and "pop" with the
parameter on the right (instead of "push" you can simply write
the name of the variable)

whilenz (...) //executes until the last result is NIL (does not
remove top of stack)
(With it, you can make an ordinary while loop:)
cond
whilenz (... cond)

editor:
DEFUN "brackets2list_delstring"
(
  eat //"("
  pushnil
  whilez (
    gettoken_del
    isopenbracket ifz (
      string2list_delstring //eat (...)
    )
    ... pushtolist ... //can add ")"
    isclosebracket ifz //what if there's no closing parenthesis?
                  //input directly in parentheses without Enter?
  )
  ... popfromlist ...
  ... swaplist ...
)
DEFUN "readlist" (readbrackets brackets2list_delstring)
DEFUN "eval_dellist" (DUP eval SWAP dellist)
loop (readlist eval_dellist printlist_dellist)

                             * * *

The obvious drawback is that the program in this language takes
at least twice as much as the Forth's, precisely because of the
storage in the list. A data structure "list" is rarely needed
for real Spectrum problems, it was chosen in order to be able to
edit the program in the process of its work.

If the project is completed, it will be possible to use it for a
program of infinite size, if all the pointers are made far
(3 bytes instead of 2). But for such large programs it is
necessary to be convenient to code...