Prolog: update initial code for denotational_semantics problems

1 files changed, 27 insertions, 0 deletions

diff --git a/prolog/problems/denotational_semantics/prog_listswap_3/common.py b/prolog/problems/denotational_semantics/prog_listswap_3/common.py
index 3ad93e8..800717e 100644
--- a/prolog/problems/denotational_semantics/prog_listswap_3/common.py
+++ b/prolog/problems/denotational_semantics/prog_listswap_3/common.py
@@ -21,3 +21,30 @@ instr174((R0,C0)-->(R0,C)) -->
 
 instr174((R0,C0)-->(R,C0)) -->
   [swap], {swap(R0,C0,R)}.'''
+
+# nothing to do in this exercise
+initial = '''\
+prog_listswap(In-->Out) -->
+    [begin], instructs((In,1)-->(Out,_)), [end].
+
+% one instruction
+instructs((R0,C0)-->(R,C)) -->
+    instr((R0,C0)-->(R,C)).
+% sequence of instructions
+instructs((R0,C0)-->(R,C)) -->
+    instr((R0,C0)-->(R1,C1)),
+    instructs((R1,C1)-->(R,C)).
+
+% "left" and "right" instructions move the cursor (but not outside list bounds)
+instr((R0,C0)-->(R0,C)) -->
+    [left],
+    { C0 > 1, C is C0 - 1 ; C0 =< 1, C is C0 }.
+instr((R0,C0)-->(R0,C)) -->
+    [right],
+    { length(R0, LenR0), (C0 < LenR0, C is C0 + 1 ; C0 >= LenR0, C is C0) }.
+
+% "swap" instruction swaps the element under cursor with the element on the left
+instr((R0,C0)-->(R,C0)) -->
+    [swap],
+    { swap(R0, C0, R) }.
+'''