Implement the following plan:

Plan: Support ALT Replicators (Issue #45)

Context

The transpiler supports replicated SEQ, PAR, and IF but not replicated ALT (ALT i = 0 FOR n). This blocks 5 out of 16 course example files. The replicated ALT dynamically creates N channel alternatives and selects whichever is ready first. Since Go's select requires statically-known cases, the codegen must use reflect.Select for runtime-variable case counts.

Changes

1. AST (ast/ast.go)

Add Replicator field to AltBlock and Declarations field to AltCase:

type AltBlock struct {
    Token      lexer.Token
    Cases      []AltCase
    Replicator *Replicator   // NEW: optional ALT i = start FOR count
}

type AltCase struct {
    // ... existing fields ...
    Declarations []Statement  // NEW: scoped declarations before channel input (e.g., BYTE ch:)
}

The Declarations field is needed because replicated ALT cases commonly have scoped variable declarations before the channel input (e.g., BYTE ch: / VAL INT X IS (j \ s):).

2. Parser (parser/parser.go)

parseAltBlock() (~line 1568): Add replicator detection after consuming ALT token, using the same pattern as parseSeqBlock()/parseParBlock()/parseIfStatement():

// Check for replicator: ALT i = start FOR count
if p.peekTokenIs(lexer.IDENT) {
    // Save state, peek ahead for =
    p.nextToken()
    if p.peekTokenIs(lexer.EQ) {
        block.Replicator = p.parseReplicator()
    } else {
        // Not a replicator — error (ALT doesn't take bare IDENT after keyword)
        p.addError("unexpected identifier after ALT")
    }
}

parseAltCase() (~line 1640): Handle scoped declarations before the channel input. When the current token is a type keyword (INT_TYPE, BYTE_TYPE, etc.) or VAL or LBRACKET (for array decls), parse it as a declaration and store in altCase.Declarations. Loop until we hit an identifier that starts a channel input.

3. Codegen (codegen/codegen.go)

Generator struct: Add needReflect bool field.

Import scanning (~line 106): Add containsAltReplicator() scanner that walks the AST checking for AltBlock nodes with non-nil Replicator. Set g.needReflect = true.

Import block (~line 204): Add "reflect" to import list when needReflect is true.

generateAltBlock() (~line 1604): When alt.Replicator != nil, generate reflect.Select code:

// Generated pattern for: ALT i = 0 FOR n  /  BYTE ch: / in[i] ? ch / body
{
    _count := <count>
    _cases := make([]reflect.SelectCase, _count)
    for _i := 0; _i < _count; _i++ {
        _cases[_i] = reflect.SelectCase{Dir: reflect.SelectRecv, Chan: reflect.ValueOf(in[_i])}
    }
    _chosen, _value, _ := reflect.Select(_cases)
    i := <start> + _chosen
    var ch byte           // from scoped declaration
    ch = _value.Interface().(byte)  // type from declaration
    // ... body ...
}

Key details: - Build reflect.SelectCase slice with Dir: reflect.SelectRecv and Chan: reflect.ValueOf(channelArray[index]) - The start expression is added to _chosen to compute the replicator variable - Scoped declarations are generated as local var statements - The receive variable is assigned from _value.Interface().(goType) where goType comes from occamTypeToGo() on the declaration type - STEP support: use counter-based pattern like PAR replicator - Handle scoped abbreviations (like VAL INT X IS (j \ s):) by generating them after the replicator variable is set

4. Tests

Parser test (parser/parser_test.go): Test that ALT i = 0 FOR n parses correctly with replicator, scoped declarations, indexed channel input, and body.

E2e test (codegen/e2e_test.go or codegen/e2e_replicator_test.go): Test replicated ALT with channel arrays — spawn N goroutines sending on channels, use replicated ALT to receive, verify correct value received.

5. Documentation (TODO.md, CLAUDE.md)

Update to reflect ALT replicator support.

Verification

go test ./...                    # all tests pass
go test ./parser -run TestAlt    # parser tests for replicated ALT
go test ./codegen -run TestE2E   # e2e tests including replicated ALT

# Real-world validation:
./occam2go -I kroc/modules/course/libsrc -D TARGET.BITS.PER.WORD=32 \
    -o /tmp/sort_pump.go kroc/modules/course/examples/sort_pump.occ

If you need specific details from before exiting plan mode (like exact code snippets, error messages, or content you generated), read the full transcript at: /home/david/.claude/projects/-home-david-projects-code-associates-occam2go/1826a3f0-b019-499a-abc6-72315cb03f4c.jsonl

Thank you. Please create a PR for this work.

Thank you. I've approved the PR. Please merge.

/exit exit

Catch you later!

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