TITLE: The Proto Interpreter for J AUTHOR: Eugene Wallingford DATE: October 19, 2013 7:38 AM DESC: ----- BODY:
(Update: Josh Grams took my comment about needing a week of work to grok this code as a challenge. He figured it out much more quickly than that and wrote up an annotated version of the program as he went along.I like finding and reading about early interpreters for programming languages, such as the first Lisp interpreter or Smalltalk-71, which grew out of a one-page proof of concept written by Dan Ingalls on a bet with Alan Kay. So I was quite happy recently to run across Appendix A from An Implementation of J, from which comes the following code. Arthur Whitney whipped up this one-page interpreter fragment for the AT&T 3B1 one weekend in 1989 to demonstrate his idea for a new APL-like like language. Roger Hui studied this interpreter for a week before writing the first implementation of J.
I think it will take me a week of hard work to grok this code, too. Whitney's unusually spare APL-like C programming style is an object worthy of study in its own right. By the way, Hui's Appendix A bears the subtitle Incunabulum, a word that means a work of art or of industry of an early period. So, I not only discovered a new bit of code this week; I also learned a cool new word. That's a good week. -----typedef char C;typedef long I; typedef struct a{I t,r,d[3],p[2];}*A; #define P printf #define R return #define V1(f) A f(w)A w; #define V2(f) A f(a,w)A a,w; #define DO(n,x) {I i=0,_n=(n);for(;i<_n;++i){x;}} I *ma(n){R(I*)malloc(n*4);}mv(d,s,n)I *d,*s;{DO(n,d[i]=s[i]);} tr(r,d)I *d;{I z=1;DO(r,z=z*d[i]);R z;} A ga(t,r,d)I *d;{A z=(A)ma(5+tr(r,d));z->t=t,z->r=r,mv(z->d,d,r); R z;} V1(iota){I n=*w->p;A z=ga(0,1,&n);DO(n,z->p[i]=i);R z;} V2(plus){I r=w->r,*d=w->d,n=tr(r,d);A z=ga(0,r,d); DO(n,z->p[i]=a->p[i]+w->p[i]);R z;} V2(from){I r=w->r-1,*d=w->d+1,n=tr(r,d); A z=ga(w->t,r,d);mv(z->p,w->p+(n**a->p),n);R z;} V1(box){A z=ga(1,0,0);*z->p=(I)w;R z;} V2(cat){I an=tr(a->r,a->d),wn=tr(w->r,w->d),n=an+wn; A z=ga(w->t,1,&n);mv(z->p,a->p,an);mv(z->p+an,w->p,wn);R z;} V2(find){} V2(rsh){I r=a->r?*a->d:1,n=tr(r,a->p),wn=tr(w->r,w->d); A z=ga(w->t,r,a->p);mv(z->p,w->p,wn=n>wn?wn:n); if(n-=wn)mv(z->p+wn,z->p,n);R z;} V1(sha){A z=ga(0,1,&w->r);mv(z->p,w->d,w->r);R z;} V1(id){R w;}V1(size){A z=ga(0,0,0);*z->p=w->r?*w->d:1;R z;} pi(i){P("%d ",i);}nl(){P("\n");} pr(w)A w;{I r=w->r,*d=w->d,n=tr(r,d);DO(r,pi(d[i]));nl(); if(w->t)DO(n,P("< ");pr(w->p[i]))else DO(n,pi(w->p[i]));nl();} C vt[]="+{~<#,"; A(*vd[])()={0,plus,from,find,0,rsh,cat}, (*vm[])()={0,id,size,iota,box,sha,0}; I st[26]; qp(a){R a>='a'&&a<='z';}qv(a){R a<'a';} A ex(e)I *e;{I a=*e; if(qp(a)){if(e[1]=='=')R st[a-'a']=ex(e+2);a= st[ a-'a'];} R qv(a)?(*vm[a])(ex(e+1)):e[1]?(*vd[e[1]])(a,ex(e+2)):(A)a;} noun(c){A z;if(c<'0'||c>'9')R 0;z=ga(0,0,0);*z->p=c-'0';R z;} verb(c){I i=0;for(;vt[i];)if(vt[i++]==c)R i;R 0;} I *wd(s)C *s;{I a,n=strlen(s),*e=ma(n+1);C c; DO(n,e[i]=(a=noun(c=s[i]))?a:(a=verb(c))?a:c);e[n]=0;R e;} main(){C s[99];while(gets(s))pr(ex(wd(s)));}