if else then

core.fth

@@ -34,7 +34,12 @@
 : +!        dup >r swap r> @ + swap ! ;
 : allot     dp +! ;
 : ,         here ! cell allot ;
+: [']       ' [ ' literal , ] ; immediate
 
 : 1+        1 + ;
 : 1-        1 - ;
 
+: if       ['] _jmp0 , here 0 , ; immediate
+: then     here swap ! ; immediate
+: else     ['] _jmp , here 0 , swap here swap ! ; immediate
+

forth.hpp

@@ -282,6 +282,16 @@ struct forth
         auto f_semic = [](auto) { *fth.here++ = 0; fth.compiling = false; };
         auto f_comm  = [](auto) { fth.sourcei = npos; };
         auto f_cell  = [](auto) { fth.push(sizeof(cell)); };
+        auto f_jmp   = [](auto) {
+            auto ptr = ++fth.ip;
+            fth.ip = *reinterpret_cast<func **>(ptr) - 1;
+        };
+        auto f_jmp0  = [](auto) {
+            auto ptr = ++fth.ip;
+
+            if (fth.pop() == 0)
+                fth.ip = *reinterpret_cast<func **>(ptr) - 1;
+        };
 
         constexpr static word w_dict {"_d", f_dict};
         constexpr static word w_add {"+", f_add, &w_dict};
@@ -308,8 +318,10 @@ struct forth
         constexpr static word w_semic {";", f_semic, &w_colon, word_base::immediate};
         constexpr static word w_comm {"\\", f_comm, &w_semic, word_base::immediate};
         constexpr static word w_cell {"cell", f_cell, &w_comm};
+        constexpr static word w_jmp {"_jmp", f_jmp, &w_cell};
+        constexpr static word w_jmp0 {"_jmp0", f_jmp0, &w_jmp};
 
-        fth.latest = &w_cell;
+        fth.latest = &w_jmp0;
         fth.end = end_value;
     }