MPL Bytecode Compiler Code

23 lines (17 with data), 258 Bytes

##LIBHDR

X=1 Y=1 C=0
.TEXT="THIS IS SOME TEXT TO EDIT"

EDIT.LINE() [
  @ [
    GOTOXY(X,Y); C=READKEY();
    C==.VK.LEFT?--X:C==.VK.RIGHT?++X
  ]
]

.() [
  CLRSCR();
  GOTOXY(1,Y);
  CPUTS(.TEXT);
  EDIT.LINE();
  WRITEN(C);
  0
]