Claude Code transcript

Yes, I'm familiar with transpilers. A transpiler (or source-to-source compiler) is a type of compiler that takes source code written in one programming language and produces equivalent source code in another language (or a different version of the same language).

Key characteristics:

• Source-to-source: Unlike traditional compilers that output machine code or bytecode, transpilers output human-readable source code
• Similar abstraction level: Typically the input and output languages operate at a similar level of abstraction

Common examples:

• TypeScript → JavaScript: The TypeScript compiler (tsc) transpiles TypeScript to JavaScript
• Babel: Transpiles modern JavaScript (ES6+) to older versions for browser compatibility
• CoffeeScript → JavaScript
• Sass/SCSS → CSS
• Cython → C: Python-like code to C for performance

Transpilers are useful for: - Using modern language features while targeting older runtimes - Writing in a preferred syntax while deploying to a required target - Gradual migration between languages - Adding type safety or other features to existing ecosystems

Is there something specific about transpilers you'd like to discuss or are you working on something related?

This is a fascinating project with some lovely computing history behind it. The "party in Putney" is a nice allusion to the connections between Rob Pike, Tony Hoare's CSP work, and the lineage from Occam through to Go.

Why this is a good fit:

The conceptual mapping between Occam and Go is remarkably clean:

Challenges to consider:

1. Indentation-sensitive parsing: Occam uses significant whitespace like Python, which adds complexity to the lexer/parser
2. Replicators: Occam's PAR i = 0 FOR n needs translation to Go loops spawning goroutines
3. Type system differences: Occam's types don't map 1:1 to Go
4. Process model: Occam processes are more rigid than goroutines
5. Arrays and slices: Different semantics

Proposed architecture:

Occam source → Lexer → Parser → AST → Code Generator → Go source

Questions for scoping:

• Are you targeting a subset of Occam (e.g., Occam 2) or the full language?
• Do you have example Occam programs you'd like to transpile?
• Is this primarily for educational/historical interest, or do you have real Occam code to migrate?

I think this is very achievable. Shall I proceed with a design, or would you like to provide more details first?

Done. The basic CLI scaffold is ready:

occam2go/
├── go.mod      # Go module (github.com/codeassociates/occam2go)
├── main.go     # CLI entry point
└── occam2go    # Built binary

The CLI currently supports: - --version - displays version (0.1.0) - -o <file> - specify output file (defaults to stdout) - Positional argument for the input .occ file

Ready for the next piece of functionality when you are.

That's a very sensible incremental approach. Starting with the foundation and building up:

Phase 1: Variables & Expressions - Lexer to tokenize Occam (handling indentation-sensitive syntax) - Parser to build an AST - Basic types: INT, BYTE, BOOL - Declarations: INT x: or INT x, y, z: - Assignment: x := 5 - Expressions: arithmetic, comparisons - SEQ blocks (which just become sequential Go code)

Phase 2: Procedures - PROC declarations → Go functions - Parameters (value and reference) - Procedure calls

Phase 3: PAR - PAR blocks → goroutines with sync.WaitGroup - Replicators like PAR i = 0 FOR n → loops spawning goroutines

Deferring channels and ALT makes sense - they add significant complexity (especially ALT → select with proper semantics), and you can demonstrate the core transpiler working without them.

One consideration: Occam's indentation-based scoping is the trickiest part of the lexer. We could either: 1. Handle it properly from the start (more work upfront, cleaner later) 2. Start with a simplified syntax for testing, then add proper indentation handling

I'd suggest option 1 since it affects everything else, but curious what you think.

Shall I begin with the lexer?

Understood. The files to commit are:

• go.mod
• main.go
• ast/ast.go
• codegen/codegen.go, codegen/codegen_test.go
• lexer/lexer.go, lexer/token.go, lexer/lexer_test.go
• parser/parser.go, parser/parser_test.go
• examples/simple.occ, examples/parallel.occ, examples/procedure.occ

You'll likely want to skip the occam2go binary.

That's an excellent point. The current tests only verify string output, not that the generated code actually works. A proper end-to-end test would:

1. Transpile Occam → Go
2. Compile the Go code
3. Run the binary
4. Verify the output

For this we need I/O. I'd suggest the simplest pragmatic approach:

Add built-in print procedures:

SEQ
  INT x, y:
  x := 10
  y := 20
  print.int(x + y)

Generates:

fmt.Println(x + y)

Implementation needed: 1. Recognize print.int, print.string, print.newline as built-in procedures 2. Generate fmt.Println(...) or fmt.Print(...) calls 3. Auto-import "fmt" when print is used 4. Add end-to-end tests that compile and run generated code

This is simpler than Occam's authentic channel-based I/O to screen/keyboard processes, but gets us to working end-to-end tests quickly. We could add proper channel I/O later.

The test would look something like:

func TestEndToEnd_Addition(t *testing.T) {
    occam := `SEQ
  INT x:
  x := 10 + 20
  print.int(x)
`
    output := transpileCompileRun(t, occam)
    if output != "30\n" {
        t.Errorf("expected 30, got %s", output)
    }
}

Does this approach work for you?

Done. Here's what was added:

Built-in print procedures: - print.int(x) - prints an integer - print.bool(x) - prints a boolean - print.string(x) - prints a string - print.newline() - prints a blank line

End-to-end tests (codegen/e2e_test.go): - 10 tests that transpile Occam → compile Go → run binary → verify output - Tests cover: arithmetic, expressions, procedures, PAR blocks, booleans, comparisons

Files changed: - codegen/codegen.go - added print builtin handling and fmt import - codegen/e2e_test.go - new end-to-end test suite - examples/print.occ - example using print - README.md - documented print procedures

The test suite now verifies the complete transpile/compile/execute workflow.

Channels are now implemented. Here's a summary:

New features: - CHAN OF <type> name: - channel declaration → make(chan <type>) - c ! x - send → c <- x - c ? y - receive → y = <-c

Files changed: - ast/ast.go - added ChanDecl, Send, Receive nodes - parser/parser.go - added channel parsing, fixed nested block handling - codegen/codegen.go - added channel code generation - parser/parser_test.go - added channel parser tests - codegen/e2e_test.go - added 3 end-to-end channel tests - examples/channel.occ - channel example - README.md - documented channels

Bug fix: - Fixed parsing of nested blocks in PAR (e.g., PAR containing multiple SEQ blocks)

End-to-end tests added: - Basic channel send/receive - Sending expressions over channels - Two-way ping-pong communication

Given what's implemented, here are my suggestions in order of impact:

1. ALT → select (High value)

Completes the core CSP primitives. ALT allows a process to wait on multiple channels and proceed with whichever is ready first.

ALT
  c1 ? x
    process1()
  c2 ? y
    process2()

→

select {
case x = <-c1:
    process1()
case y = <-c2:
    process2()
}

This is powerful but has subtleties (guards, priority ALT, ALT with timeout).

2. Replicators (High value)

PAR i = 0 FOR n spawns n parallel processes. Very common in Occam for data parallelism.

PAR i = 0 FOR 4
  worker(i)

→

for i := 0; i < 4; i++ {
    i := i // capture
    go func() { worker(i) }()
}

Also useful: SEQ i = 0 FOR n for sequential loops.

3. WHILE loops (Medium value)

Parser has partial support. Straightforward to complete.

WHILE x > 0
  x := x - 1

4. Input procedures (Medium value)

Complement to print.* for interactive programs: - input.int(x) → fmt.Scan(&x)

5. Arrays (Lower priority for now)

Important but adds complexity. Could be deferred.

My recommendation: ALT would be the most impactful, as it completes the CSP model and enables proper concurrent program patterns. Replicators would be a close second for practical utility.

Which direction interests you?

Given what's implemented, I'd recommend WHILE loops as the next step:

Why WHILE: 1. The parser already has partial support - it should be quick to complete 2. Maps directly to Go's for condition { } loop 3. Immediately enables iterative programs 4. Easy to test end-to-end

WHILE x > 0
  SEQ
    print.int(x)
    x := x - 1

→

for x > 0 {
    fmt.Println(x)
    x = x - 1
}

After WHILE, I'd suggest Replicators: These are essential for idiomatic Occam parallel programming:

PAR i = 0 FOR 4
  worker(i)

→

var wg sync.WaitGroup
wg.Add(4)
for i := 0; i < 4; i++ {
    i := i // capture
    go func() {
        defer wg.Done()
        worker(i)
    }()
}
wg.Wait()

WHILE would be a quick win; replicators would unlock more powerful parallel patterns.

Shall I proceed with WHILE?

Replicators are now implemented. Here's a summary of the changes:

Parser updates (parser/parser.go): - Modified parseSeqBlock and parseParBlock to detect replicator syntax (when an identifier follows SEQ/PAR and is followed by =) - Added parseReplicator() function that parses variable = start FOR count

Code generator updates (codegen/codegen.go): - Replicated SEQ generates a for loop: for i := start; i < start + count; i++ { ... } - Replicated PAR generates goroutines in a loop with proper loop variable capture to avoid closure issues

Tests added: - Parser tests for replicated SEQ and PAR - E2E tests covering: - Basic replicated SEQ - Replicated SEQ with variable count - Replicated SEQ with non-zero start offset - Replicated SEQ computing a sum - Replicated PAR with channel communication

Example (examples/replicator.occ):

SEQ i = 0 FOR 5
  print.int(i)

Outputs: 0 1 2 3 4 (one per line)

Based on the README and typical Occam usage, here are the remaining unimplemented features:

Currently listed as not implemented: 1. IF (guarded commands) - Occam's IF is different from most languages; it's a list of guarded choices where the first true guard executes. Unlike Go's if/else, if no guard is true, the process stops (STOP semantics).

1. Arrays - [10]INT arr: declarations, indexed access arr[i], and array slicing. This would enable more realistic programs.

Other useful features not yet covered:

1. PRI ALT - Priority alternation where earlier cases are preferred over later ones. Would require a different code generation strategy than simple select.

2. Replicated ALT - ALT i = 0 FOR n to select from an array of channels.

3. CASE statement - Pattern matching on values.

4. Timers - TIMER t: and t ? now for timing operations, maps to Go's time package.

Recommendation: Arrays would likely provide the most value, as they're fundamental for non-trivial programs and would combine well with the replicators you just implemented (e.g., SEQ i = 0 FOR 10 operating on arr[i]). IF would also be practical since it's a core control structure.

Would you like me to implement arrays or IF next?