[joe_the_user]

Here is a prolog in interpreter in c++.

http://www.cl.cam.ac.uk/~am21/research/funnel/prolog.c

A list of tiny, powerful interpreters would be good.

[tromp]

The (binary) lambda calculus interpreter at http://www.ioccc.org/2012/tromp/tromp.c, documented in http://www.ioccc.org/2012/tromp/hint.html is as tiny (and incomprehensible) as it gets, but does all of input, recursive descent parsing, translation to bytecode, lazy evaluation (call-by-need to be precise), garbage collection, and output.

[joe_the_user]

Would it be possible to simply a have a normally formatted version of the code?

[tromp]

A slightly less obfuscated and somewhat more normally formatted version:

  #include <stdio.h>
  #include <stdlib.h>
  #include <assert.h>
  enum {I,O,V,A,L};
  int n=44,i,c,T[M]={L,A,8,A,2,  V,0,L,L,V,
      A,30,L,A,2,V,0,L,A,5,A,7,L,V,0,O,
      A,14,L,A,2,V,0,L,A,5,A,2,  V,0,O,O,A},b,s;
  typedef struct _{int t,r; struct _*e,*n;} C;C*e,*f,*l,*S[M];
  void x(int l,int u){for(;l<=u;T[n++]=T[l++]);}
  int g(){i--||(i=b,c=getchar());return c>>i&1;}
  void d(C*l){!l||--l->r||(d(l->e),d(l->n),l->n=f,f=l);}
  int p(int m){if(g()){for(T[n++]=V;g();T[n]++);n++;}else
            T[m]=n++&&g()?(T[m+1]=p(++n),A):L,p(n);return n-m;}
  int main(int t){char o;
   b=t>1?0:7;T[43]=p(n);i=0;
   for(t=b?10:26;;)switch(T[t]){
    case I: g();i++;assert(n<M-99);if(~c&&b){x(0,6);for(T[n-5]=96;i;T[n++]=!g())
             x(0,9);}x(c<0?7:b,9);T[n++]=!b&&!g();break;
    case O: t=b+t>42?(o=2*o|t&1,28):(putchar
                (b?o:t+8),fflush(stdout),b?12:28);break;
    case V: l=e;for(t=T[t+1];t--;e=e->n);
                     t=e->t;(e=e->e)&&e->r++;d(l);break;
    case A: t+=2;f||(f=calloc(1,sizeof(C)));assert(f&&  s<M);S[s++]=l=f;f=l->n;
            l->r=1;l->t=t+T[t-1];(l->e=e)&&e->r++;break;
    case L: if(!s--)return 0;S[s]->n=e;e=S[s];t++;break;
   }
   return T[t+2];
  }

[e12e]

I loved the "How I start: Nim"-article that was posted to hn a while back. Interpreter, then compile-to-nim implementation of brainfuck. All in a handful of code:

https://news.ycombinator.com/item?id=9021244