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AILANG Debugging Guide

Complete guide to debugging AILANG with environment variables and tools.

Debug Flags​

AILANG provides environment variables for verbose debugging and strict error checking.

DEBUG_STRICT=1 - Catch Silent Failures Early​

What it does: Makes incomplete switch statements and unhandled cases fail loudly with panic instead of silently returning unchanged values.

When to use:

  • During development of new compiler passes
  • When debugging AST traversal code
  • To catch missing cases in switch statements
  • In CI to enforce completeness

Example:

# Normal mode - unhandled cases return unchanged (silent failure)
$ ailang run test.ail
# ✓ May complete successfully even with bugs!

# Strict mode - unhandled cases panic immediately
$ DEBUG_STRICT=1 ailang run test.ail
panic: cloneExpr: unhandled node type *core.Record (NodeID 42).
    Add a case for this type or explicitly mark as unsupported.
# ✓ Bug caught immediately!

Affected functions (as of v0.4.1):

  • internal/pipeline/specialize.go:
    • cloneExpr() - Cloning during monomorphization
    • specializeExpr() - Specializing expressions

DEBUG_MONO_VERBOSE=1 - Monomorphization Tracing​

What it does: Logs detailed information about monomorphization (polymorphic function specialization).

When to use:

  • Debugging type substitution issues
  • Understanding which functions are specialized
  • Tracking down operator re-linking problems

Example:

$ DEBUG_MONO_VERBOSE=1 ailang run --entry main --debug-compile test.ail
[DEBUG_MONO_VERBOSE] Found lambda, type=α2 -> α2 -> α2, isPoly=true
[DEBUG_MONO_VERBOSE] lambda type from CoreTI: α2 -> (α2 -> α2)
[DEBUG_MONO_VERBOSE] extracted paramTVars: [α2]
[DEBUG_MONO_VERBOSE] typeSubst built: map[α2:float]
[DEBUG_MONO_VERBOSE] Cloning DictApp: method=gt
[DEBUG_MONO_VERBOSE]   Original DictRef: class=Ord, type=Int, NodeID=15
[DEBUG_MONO_VERBOSE]   Cloned DictRef: class=Ord, type=float, NodeID=42

DEBUG_OPERATOR_LOWERING=1 - Operator Resolution Tracing​

What it does: Logs operator lowering decisions (BinOp/DictApp → Intrinsic).

When to use:

  • Debugging operator dispatch issues
  • Understanding which builtin is selected
  • Tracking type-guided operator selection

DEBUG_PARSER=1 - Parser Token Tracing​

What it does: Shows ENTER/EXIT for parser functions with current/peek tokens.

When to use:

  • Debugging parser token position issues
  • Understanding parser flow
  • Tracking token consumption

Example:

$ DEBUG_PARSER=1 ailang run test.ail
[ENTER parseType] cur=IDENT(int) peek=,
[EXIT parseType] cur=IDENT(int) peek=,
[ENTER parseExpression] cur=IDENT(x) peek=+
[EXIT parseExpression] cur=IDENT(x) peek=+

Combining Debug Flags​

Recommended combinations:

# Development mode - catch bugs early + verbose output
$ DEBUG_STRICT=1 DEBUG_MONO_VERBOSE=1 ailang run test.ail

# CI mode - strict checking only (no verbose output)
$ DEBUG_STRICT=1 make test

# Deep debugging - all flags
$ DEBUG_STRICT=1 DEBUG_MONO_VERBOSE=1 DEBUG_OPERATOR_LOWERING=1 ailang run --debug-compile test.ail

# Parser debugging
$ DEBUG_PARSER=1 ailang run test.ail

Quick Reference Table​

Environment Variables​

FlagPurposeUse WhenOutput
DEBUG_STRICT=1Fail loudly on unhandled casesDevelopment, CIPanics with diagnostic
DEBUG_MONO_VERBOSE=1Monomorphization tracingType issuesSpecialization details
DEBUG_OPERATOR_LOWERING=1Operator resolutionDispatch issuesBuiltin selection
DEBUG_PARSER=1Token position tracingParser bugsToken flow

CLI Flags​

FlagPurposeUse When
--debug-compileShow compilation phases/timingPerformance issues
--debug-typesType inference debug outputType mismatch errors
--debug-types --node NFilter to specific node IDInvestigating specific node
--traceExecution tracingRuntime debugging

CLI Debug Flags​

In addition to environment variables, AILANG CLI provides debug flags:

# Show compilation phases
ailang run --debug-compile file.ail

# Enable execution tracing
ailang run --trace file.ail

# Type-check only (no execution)
ailang check file.ail

# Show module interface
ailang iface mymodule

# Type inference debugging (v0.5.11+)
ailang run --debug-types file.ail
ailang run --debug-types --node 42 file.ail  # Filter to specific node

--debug-types - Type Inference Debugging (v0.5.11+)​

What it does: Shows detailed type inference information including:

  • Substitution map (type variable → resolved type)
  • Constraints (type class constraints and their resolution status)
  • CoreTI entries (type information for each Core AST node)
  • Origins/provenance (where each type came from)

When to use:

  • Understanding why a type was inferred
  • Debugging type mismatch errors
  • Investigating constraint resolution
  • Verifying type annotations are applied correctly
  • Answering "why does this have type X?"

Example:

$ ailang run --debug-types --caps IO examples/debug_types_demo.ail
=== Type Inference Debug ===

[Substitution Map]
  α1 → α2
  α5 → α7 → α11 (CHAIN)
  α6 → α10
  α8 → α6 -> α7 (direct)

[Constraints]
  Added:
    Num α1 at node 9
    Num α3 at node 14
    Fractional α41 at node 60
  Resolved:
    Num Int →  at node 9
    Fractional Float →  at node 60

[CoreTI Entries]
  NodeID 1: string -> () ! {IO}
  NodeID 9: int
    Constraint: Num → add
  NodeID 60: float
    Constraint: Fractional (resolved)
  ...

Filtering by node:

# Show type info only for node ID 9
$ ailang run --debug-types --node 9 --caps IO examples/debug_types_demo.ail

[Constraints]
  Added:
    Num α1 at node 9
  Resolved:
    Num Int →  at node 9

[CoreTI Entries]
  NodeID 9: int
    Constraint: Num → add

Output sections:

  • Substitution Map: Shows type variable substitutions (α → β → int means α resolved to β which resolved to int)
  • Constraints: Type class constraints (Num, Eq, Ord) and whether they're resolved
  • CoreTI Entries: Every Core AST node's inferred type, constraints, and origins
  • Origins: Where the type came from (annotation, literal, inferred, defaulted, etc.)

Understanding Origins (Provenance)​

The Origins: section answers "why does this expression have this type?" Each origin shows:

  • Kind: How the type was determined (annotation, literal, inferred, defaulted, from_use, from_pattern)
  • Note: Human-readable explanation
  • Location: Source file:line:column when available

Origin kinds:

KindMeaningExample
annotationExplicit type annotationlet x: int = 42
literalInferred from literal value3.14 → float
inferredCreated during type inferenceFresh type variable α
defaultedType variable defaultedNum α defaulted to int
from_useInferred from call siteFunction applied to int
from_patternInferred from pattern matchSome(x) binds x to inner type

Example with multiple origins:

NodeID 42: int
  Raw: α1
  Resolved: int
  Origins:
    - inferred: fresh type variable
    - defaulted: defaulted to int (Num constraint)

This shows that node 42 started as a type variable α1, then was defaulted to int because of a Num constraint.

Troubleshooting Workflows​

Scenario 1: "Why is my float becoming int?"​

Symptom: You expected float but got int arithmetic.

-- Problem: add(3.14)(2.71) gives unexpected result
let add = \x. \y. x + y
let result = add(3.14)(2.71)  -- Expected: 5.85, got: 5?

Debug workflow:

$ ailang run --debug-types myfile.ail

What to look for:

[CoreTI Entries]
  NodeID 5: int -> int -> int    -- The add function got type int!
    Constraint: Num → add
    Origins:
      - inferred: fresh type variable
      - defaulted: defaulted to int (Num constraint)  -- HERE'S THE PROBLEM

Root cause: The Num constraint defaulted to int before the float literals were seen.

Fix: Add type annotations:

let add: float -> float -> float = \x. \y. x + y

Scenario 2: "Type mismatch at line X"​

Symptom: Error says types don't match but you're not sure why.

Error: type mismatch at line 15: expected int, got α42

Debug workflow:

$ ailang run --debug-types --node 42 myfile.ail

What to look for:

NodeID 42: α42
  Origins:
    - inferred: fresh type variable

Root cause: Node 42 is still a type variable (α42) - it was never unified with a concrete type.

Fix: Check that the expression at node 42 is actually used with concrete types, or add an annotation.

Scenario 3: "Which operator is being called?"​

Symptom: x + y behaves unexpectedly for your types.

Debug workflow:

$ ailang run --debug-types myfile.ail | grep -A2 "Constraint: Num"

What to look for:

  NodeID 9: int
    Constraint: Num → add     -- Shows which method resolved
  NodeID 14: int
    Constraint: Num → mul     -- mul was selected for *

The → add shows the constraint resolved to the add method. If it says (resolved) without a method, the constraint was satisfied but method selection may differ.

Scenario 4: "Understanding polymorphic function types"​

Symptom: Function type shows type variables (α, β) instead of concrete types.

$ ailang run --debug-types myfile.ail

What to look for:

NodeID 31: α22
  Origins:
    - inferred: fresh type variable
NodeID 35: α22
  Origins:
    - inferred: fresh type variable

Interpretation: Multiple nodes share the same type variable (α22), meaning they must have the same type. This is polymorphism working correctly - the type will be specialized at each call site.

Problem: Type Inference Issues​

# 1. Use --debug-types to see all type information (v0.5.11+)
ailang run --debug-types problematic.ail

# 2. Filter to specific node if you know the ID
ailang run --debug-types --node 42 problematic.ail

# 3. Check types at each phase
ailang check problematic.ail

# 4. Enable monomorphization debugging
DEBUG_MONO_VERBOSE=1 ailang run --debug-compile problematic.ail

# 5. Check operator resolution
DEBUG_OPERATOR_LOWERING=1 ailang run problematic.ail

Problem: Parser Not Recognizing Syntax​

# 1. Trace token flow
DEBUG_PARSER=1 ailang run problematic.ail

# 2. Check for lexer issues
# (Lexer never generates NEWLINE tokens!)

# 3. Use parser-developer skill for conventions

Problem: Silent Failures in Compiler Pass​

# 1. Enable strict mode
DEBUG_STRICT=1 ailang run problematic.ail

# 2. Will panic on unhandled cases with diagnostic
# 3. Add missing cases to switch statement

Problem: Unexpected Operator Behavior​

# 1. Check type defaulting (Num typeclass defaults to int)
ailang check problematic.ail

# 2. Add type annotations
# let add: float -> float -> float = \x. \y. x + y

# 3. Enable operator tracing
DEBUG_OPERATOR_LOWERING=1 ailang run problematic.ail

Keeping ailang Up to Date​

After making code changes to the ailang binary:

make quick-install  # Fast reinstall (recommended for development)
# OR
make install        # Full reinstall with version info

Important: The ailang command in your PATH points to the system install, NOT the local build. Always run make install or make quick-install after building to update the system binary.

For local testing without install:

./bin/ailang <command>  # Use local build directly

Development Tools​

# Code quality
make lint                 # Run golangci-lint
make fmt                  # Format all Go code
make vet                  # Run go vet
make test-coverage        # Run tests with coverage

# File organization
make check-file-sizes     # Fails CI if any file >800 lines
make report-file-sizes    # Show files >500 lines

# Documentation
make doc PKG=<package>    # Show package documentation

See Also​