Gradual Programming: Bridging the Semantic Gap

1. Gradual Programming: Bridging the Semantic Gap Bor-Yuh Evan Chang Amer Diwan Jeremy G. Siek University of Colorado, Boulder PLDI FIT 2009

2. Have you noticed a time where your program is not optimized where you expect? Observation: A disconnect between programmer intent and program meaning “I need a map data structure” Load class file Run class initialization Create hashtable semantic gap Problem: Tools (IDEs, checkers, optimizers) have no knowledge of what the programmer cares about … hampering programmer productivity, software reliability, and execution efficiency Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

3. Example: Iteration Order class OpenArray extends Object { private Double data[]; public boolean contains(Object lookFor) { for (i = 0; i < data.length; i++) { if (data[i].equals(lookFor)) return true; } return false; } } Must specify an iteration order even when it should not matter • class OpenArray extends Object { • private Double data[]; • public boolean contains(Object lookFor) { • for (i = 0; i < data.length; i++) { • if (data[i].equals(lookFor)) return true; • } • return false; • } • } Compiler cannot choose a different iteration order (e.g., parallel) Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

4. Wild and Crazy Idea: Use Non-Determinism • Programmer starts with a potentially non-deterministic program • Analysis identifies instances of “under-determinedness” • Programmer eliminates “under-determinedness” “over-determined” just right Question: What does this mean? Is it “under-determined”? Response: Depends, is the iteration order important? • class OpenArray extends Object { • private Double data[]; • public boolean contains(Object lookFor) { • for (i = 0; i < data.length; i++) { • if (data[i].equals(lookFor)) return true; • } • return false; • } • } • class OpenArray extends Object { • private Double data[]; • public boolean contains(Object lookFor) { •  i  0 .. data.length-1 { • if (data[i].equals(lookFor)) return true; • } • return false; • } • } starting point “under-determined” Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

5. Let’s try a few program variants • public boolean contains(Object lookFor) { • for (i = 0; i < data.length; i++) { • if (data[i].equals(lookFor)) return true; } • return false; • } Do they compute the same result? Approach: Try to verify equivalence of program variants up to a specification • public boolean contains(Object lookFor) { • for (i = data.length-1; i >= 0; i--) { • if (data[i].equals(lookFor)) return true; } • return false; • } Yes  Pick any one No  Ask user • public boolean contains(Object lookFor) { • parallel_for (0, data.length-1) i => { • if (data[i].equals(lookFor)) return true; } • return false; • } What about here? Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

6. Surprisingly, analysis says no. Why? Exceptions! Need user interaction to refine specification that captures programmer intent a.data = null a.contains( ) left-to-right iteration returns true right-to-left iteration throws NullPointerException Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

7. Scary Proposal? • “Fix semantics per program”: Abstract constructs with many possible concrete implementations • Apply program analysis to find inconsistent implementations • Interact with the user to refine the specification • Language designer role can enumerate the possible implementations Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming

8. Bridging the Semantic Gap “I need a map data structure” “Looks like iterator order matters for your program” “Yes, I need iteration in sorted order” “Let’s use a balanced binary tree (TreeMap)” Bor-Yuh Evan Chang, Amer Diwan, and Jeremy G. Siek - Gradual Programming