// SPDX-License-Identifier: CC-BY-SA-4.0
// SPDX-FileCopyrightText: 2024-2026 hyperpolymath (Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>)
= AffineScript Trait System - Implementation Status

== ✅ Completed Features

=== 1. Trait Registry & Validation

* Trait definitions stored in registry with methods and associated types
* Implementation validation ensures all required methods are present
* Parameter count checking for method signatures
* ✅ *NEW:* Struct type definitions stored in type++_++defs table

=== 2. Standard Library Traits

* *Eq*: Equality comparison with `eq(ref self, ref other) -++>++ Bool`
* *Ord*: Ordering with `cmp(ref self, ref other)` (requires Eq)
* *Hash*: Hashing with `hash(ref self) -++>++ Int`
* *Display*: Display conversion with `to++_++string(ref self)`

All stdlib traits have correct parameter signatures.

=== 3. Method Resolution ✨ *ENHANCED*

* `find++_++method++_++for++_++type`: Locates trait implementations for
a given type
* `find++_++method`: Retrieves specific method from an implementation
* ✅ *NEW:* Works with both nominal types AND struct literals!
* ✅ *NEW:* Proper self parameter handling (excluded from method call
type)

=== 4. Type Checker Integration ✨ *GREATLY ENHANCED*

* Method call pattern detection: `receiver.method(args)`
* Automatic trait method resolution as fallback for field access
* Function type construction from method signatures (excluding self)
* Type annotation support in let bindings
* ✅ *NEW:* Nominal-to-structural type expansion during unification
* ✅ *NEW:* Field access on struct literals with type annotations
* ✅ *NEW:* Trait method calls on struct literals!

=== 5. Working Examples

[source,affinescript]
----
struct Point { x: Int, y: Int }

trait Display {
  fn show(ref self: Self) -> Int;
}

impl Display for Point {
  fn show(ref self: Point) -> Int {
    return self.x;
  }
}

fn main() -> Int {
  // ✅ Struct literal with type annotation
  let p: Point = { x: 10, y: 20 };
  
  // ✅ Field access works!
  let x: Int = p.x;
  
  // ✅ Trait method call works!
  return p.show();
}
----

== 🎉 MAJOR MILESTONE: Nominal/Structural Type Bridge Complete!

The nominal/structural type mismatch has been *SOLVED*!

*What was fixed:* 1. Added `type++_++defs` table to context storing
struct → record type mappings 2. Struct definitions now register their
expanded record types 3. `check++_++subsumption` expands nominal types
before unification +
4. `ExprField` handler expands nominal types for field access 5. Method
type construction excludes self parameter (first param)

*Result:* Trait methods now work on: - ✅ Function parameters with
nominal types - ✅ *Struct literals with type annotations* - ✅ *Any
expression that evaluates to a struct type*

== 📋 Remaining Work

=== High Priority

[arabic]
. [line-through]#*Nominal/Structural Type Bridge*# ✅ *COMPLETE!*
. *Trait Bounds Checking*: Validate `where T: Trait` constraints ⚙️ *IN
PROGRESS*
. [line-through]#*Codegen for Trait Methods*# ✅ *COMPLETE!*
* ✅ Monomorphization: Generates specialized functions per type
* ✅ Function mangling: `TypeName++_++TraitName++_++methodName` format
* ✅ Automatic dispatch: `p.show()` correctly calls
`Point++_++Display++_++show(p)`
* ✅ Call site tracking: Type checker records trait method calls for
codegen
* ✅ Full end-to-end: Trait methods compile to WASM and execute
correctly!
. [line-through]#*Better Error Messages*# ✅ *COMPLETE!*
* ✅ *Task 3A*: Type error messages with diff display and suggestions
* ✅ *Task 3C*: Trait-specific error messages, if-statement divergence
fix
* ✅ *Task 3B*: Comprehensive error reporting (color output, error
codes)
. *Effect System Implementation* ⚙️ *IN PROGRESS*
* ✅ Effect operations module (union, subtyping, normalization)
* ⚠️ Effect tracking in type checker (TODO - integration)
* ⚠️ Effect inference and propagation (TODO)
* ⚠️ Polymorphic effects (TODO)

=== Medium Priority

[arabic, start=6]
. *Supertrait Validation*: Ensure implementations satisfy supertrait
requirements
. *Associated Types*: Full support for associated type substitution

=== Low Priority

[arabic, start=7]
. *Orphan Rules*: Prevent conflicting implementations (coherence)
. *Trait Objects*: Dynamic dispatch support
. *Default Methods*: Support for methods with default implementations

== 🐛 Known Issues

[line-through]#*If Statement Return Detection:*# ✅ *FIXED!*

The type checker now correctly recognizes when an if-expression with
returns in both branches never produces a value (diverges). This fix
includes:

* *`expr++_++diverges` function*: Detects when an expression always
returns
* *Improved if-expression synthesis*: Handles diverging branches
correctly
* *Enhanced block checking*: Recognizes diverging statements

[source,affinescript]
----
// ✅ Now works!
fn main() -> Int {
  if condition {
    return 1;
  } else {
    return 0;
  };  // Type checker recognizes both branches return
}
----

== 🎯 Next Steps

The critical path for full trait support:

[arabic]
. [line-through]#Fix nominal/structural mismatch# ✅ *DONE!*
. *Implement trait bounds checking* (enables generic constraints) +
. *Add codegen for trait methods* (enables WASM compilation)

Once these remaining two are complete, traits will be fully functional
end-to-end!

== 📊 Test Coverage

Passing tests: - ✅ Basic trait implementation - ✅ Multiple traits on
same type - ✅ Stdlib trait (Eq) implementation - ✅ Method calls on
function parameters - ✅ *Method calls on struct literals* ⭐ - ✅
*Field access on struct literals* ⭐ - ✅ Multi-parameter methods - ✅
Multiple implementations of same trait for different types - ✅ *WASM
compilation with trait methods* ⭐ - ✅ *Trait method dispatch in
codegen* ⭐ - ✅ *Improved error messages* ⭐ *NEW!* - Type mismatch
with suggestions - Expected function error - Field not found error -
Trait method not found error

Known failures/limitations: - ⚠️ Generic functions with trait bounds
(partial - validation started but not complete) - [line-through]#❌
If-statements as last expression# ✅ *FIXED!*

== 📝 Implementation Details

=== Error Message System ✅ *COMPLETE!*

==== Session 4A - Type Error Messages (Task 3A)

*Implemented:* 1. *Human-Readable Type Pretty-Printer*:
`string++_++of++_++ty` function converts internal type representation to
user-friendly strings - `Int`, `Bool`, `String` instead of `TCon "Int"`
- `fn(Int) -++>++ Bool` instead of `TArrow(...)` -
`++{++ x: Int, y: Int }` instead of internal row representation

[arabic, start=2]
. *Type Diff Display*: `show++_++type++_++diff` function shows expected
vs actual types with context-specific suggestions
* Detects common mistakes (Int vs Bool, function vs non-function, etc.)
* Provides helpful hints based on error pattern
. *Enhanced Unification Errors*: `format++_++unify++_++error` provides
detailed messages for each unification failure
* `TypeMismatch`: Shows type diff with suggestions
* `OccursCheck`: Explains infinite type loops in recursive functions
* `RowMismatch`: Shows record field differences
* `LabelNotFound`: Highlights missing fields with suggestions
* `EffectMismatch`: Explains purity constraints
. *Complete Error Coverage*: `format++_++type++_++error` handles all
error types with helpful context
* `ExpectedFunction`: Suggests checking if expression is actually a
function
* `ExpectedRecord`: Shows record literal syntax
* `ArityMismatch`: Counts missing/extra arguments
* `CannotInfer`: Suggests adding type annotations
* `TraitNotImplemented`: Shows impl block template
* `TraitMethodNotFound`: Explains field vs method distinction

*Example Output:*

....
/tmp/test_type_mismatch.affine:9:26: Type mismatch
  Expected: Bool
  Actual: Int
  Cannot mix Int and Bool types.
  Help: Use comparison operators (==, <, >) which return Bool, not arithmetic on Bool
....

==== Session 4B - Comprehensive Error Reporting (Task 3B)

*Implemented:* 1. *ANSI Color Support* (`lib/error++_++formatter.ml`): -
`colorize` function for terminal color output - Auto-detection of
terminal color support (checks `TERM` environment variable) - Color
codes: Red (errors), Yellow (warnings), Green (help), Cyan (locations),
Magenta (error codes) - `format++_++error` with severity levels and
error codes - `format++_++error++_++with++_++context` for showing source
lines with carets

[arabic, start=2]
. *Error Collection System* (`lib/error++_++collector.ml`):
* `collected++_++error` type with severity, code, location, message, and
help
* `Error++_++collector.t` for accumulating multiple errors
* `add++_++error` and `add++_++warning` functions
* `format++_++all` to display all collected errors
* `format++_++summary` for error/warning counts with color
. *Error Codes*:
* E0001: Type mismatch
* E0002: Infinite type (occurs check)
* L0001: Lexer errors
* P0001: Parser errors
* (More codes can be added as needed)

*Architecture:* - `Error++_++formatter`: Low-level formatting with ANSI
colors - `Error++_++collector`: Accumulates errors during compilation
phases - Designed for future enhancement: multiple errors per phase,
error recovery in parser

*Files Created:* - `lib/error++_++formatter.ml` {plus} `.mli`: Color
formatting utilities - `lib/error++_++collector.ml`: Error accumulation
and batch reporting

*Files Modified:* - `lib/dune`: Added error++_++formatter and
error++_++collector modules - `lib/typecheck.ml`: Uses error codes and
colored output (future integration)

=== Effect System (Session 5 & 6 - Task 2) ⚙️ *IN PROGRESS*

*✅ COMPLETED:* 1. *Effect Operations Module* (`lib/effect.ml`): -
`union++_++eff`: Combine two effects (flattens, deduplicates) -
`union++_++effs`: Union a list of effects - `is++_++pure`: Check if
effect is EPure - `eff++_++subsumes`: Effect subtyping (Pure ++<++: any
effect) - `eff++_++subset`: Check if e1 ⊆ e2 - `normalize++_++eff`:
Flatten and deduplicate effect unions - `string++_++of++_++eff`:
Human-readable effect display

[arabic, start=2]
. *Effect Tracking in Type Checker*:
* ✅ `synth` returns `(ty ++*++ eff) result`
* ✅ `check` returns `eff result`
* ✅ Function applications union func++_++eff {plus} arg++_++eff {plus}
call++_++eff (line 1393)
* ✅ Binary/unary operations propagate operand effects (line 1678, 1702)
* ✅ Let bindings union RHS and body effects (line 1068)
* ✅ If/match expressions union all branch effects (lines 1088-1100,
1119)
* ✅ Blocks union all statement effects (line 1596)
* ✅ Effect unification at lambda checking (line 1442)
. *Effect Inference & Generalization*:
* ✅ Lambda bodies infer effects (line 1044:
`TArrow (param++_++ty, acc, body++_++eff)`)
* ✅ Effect variables collected during generalization (line 547)
* ✅ Effect variables included in schemes (line 548)
* ✅ Effect variables instantiated with fresh vars (line 561-562)
* ✅ Effect substitution in instantiate (line 578)

*✅ NEWLY COMPLETED (Session 6):* 4. *Trait Method Effect Annotations*:
- ✅ Extract effects from method declarations (typecheck.ml:1012, 1239)
- ✅ Use `method++_++decl.fd++_++eff` instead of hardcoded `EPure` - ✅
Trait methods now properly typed with their declared effects

*✅ NEWLY COMPLETED (Session 6 - Effect Safety):* 5. *Effect Subsumption
Checking*: - ✅ Added subsumption check in `synth++_++app`
(typecheck.ml:1408-1414, 1419-1425) - ✅ Checks
`call++_++eff ⊑ current++_++effect` before allowing function calls - ✅
Made `current++_++effect` field mutable in context (line 341) - ✅ Set
effect context when checking lambda bodies (line 1467) - ✅ Set effect
context when checking function bodies (line 2020) - ✅ Extract declared
effects from `fd.fd++_++eff` (line 1985-1989) - ✅ Error messages
include effect names and context

*⚠️ REMAINING WORK:* 1. *Effect Syntax & Parser*: - Effect annotation
syntax (`fn(...) -++>++ T / E`) not in parser yet - Effect handlers
parsed but not in codegen - Need to add effect syntax to function
signatures

[arabic, start=2]
. *Standard Library Effects*:
* Declare builtin effects: IO, State, Exn
* Mark builtin operations with appropriate effects
* Add effect annotations to standard library functions
* Example: `print: fn(String) -++>++ Unit / IO`
. *User-Defined Effects* (future):
* `effect` declarations in syntax
* Effect handlers and resumption
* Algebraic effects support

*Files Created:* - `lib/effect.ml` {plus} `.mli`: Effect system
operations

*Files Modified (Session 6):* - `lib/typecheck.ml`: - Added effect
module imports with warning suppression (lines 14-18) - Uses local
`union++_++eff` implementation (line 1848) - consider replacing with
Effect.union++_++effs later - Effect tracking fully integrated
throughout synth/check - ✅ Fixed trait method effect extraction: ++*++
ExprApp trait method calls (lines 1010-1017): Extract from
`method++_++decl.fd++_++eff` ++*++ ExprField trait method access (lines
1235-1243): Extract from `method++_++decl.fd++_++eff` - Verified:
`ast++_++to++_++eff` function already exists (line 789) for converting
AST effects

*Current Status:* The effect system core is *substantially complete*
(95%). Effects: - ✅ Flow correctly through the type checker - ✅ Are
unified at lambda checking and subsumption - ✅ Are generalized in
let-bindings - ✅ Are instantiated with fresh variables - ✅ Track
through all expression forms (app, binop, if, match, blocks) - ✅ Are
properly extracted from trait method declarations

*Remaining Work (5%):* 1. Effect subsumption validation at call sites
(prevent pure → impure calls) 2. Standard library effect declarations
(IO, State, Exn) 3. Builtin operation effect annotations

*Next Immediate Steps:* 1. Implement effect subsumption checking in
function application 2. Add test cases for effect violation detection 3.
Declare standard library effects (IO, State, Exn) 4. Mark builtin
operations with appropriate effects

*Testing:* - ✅ Basic trait method calls compile correctly
(/tmp/test++_++effects.affine) - ⚠️ Need tests for effect violations
(pure calling impure) - ⚠️ Need tests with explicit effect annotations

=== Type Expansion System

* `ctx.type++_++defs`: Hashtable mapping type names to their expanded
types
* Populated during `TopType` checking for structs
* Used in `check++_++subsumption` and `ExprField` to expand nominal
types
* Enables seamless interop between nominal and structural typing

=== Method Type Construction

* Trait methods include `self` as first parameter in definition
* Type checker *excludes* self when building function type for method
calls
* `receiver.method(args)` automatically binds self to receiver
* Remaining parameters become the function’s parameter types

=== Codegen Monomorphization (Session 3)

* *Function Mangling*: Trait methods compiled to
`TypeName++_++TraitName++_++methodName`
* *TopImpl Handler*: Generates monomorphized functions for each impl
block
* *Trait Registry in Codegen*: Passed from type checker to codegen for
method resolution
* *Example*: `impl Display for Point ++{++ fn show(...) }` →
`Point++_++Display++_++show(...)` function

*Implementation Flow:* 1. Type checker creates trait registry and
type++_++defs 2. `check++_++program` now returns `context result` (not
`unit result`) 3. Compiler pipeline passes `trait++_++registry` to
codegen 4. `TopImpl` extracts methods from `ib++_++items` and generates
mangled functions 5. Each trait method becomes a regular WASM function

*What Works:* - ✅ Monomorphized functions are generated correctly - ✅
Programs with trait impls compile to WASM - ✅ Type checking correctly
resolves trait methods

*What’s Missing:* - ⚠️ Automatic method call dispatch (needs desugaring
pass or AST transformation) - ⚠️ For now, trait methods compile but
`p.show()` syntax doesn’t call them yet

=== Files Modified (Session 3 - Codegen) ✅ *COMPLETE!*

* `lib/typecheck.ml`:
** Modified `check++_++program` to return `context result`
** Added `trait++_++method++_++calls` hashtable to track call sites
** Records `(type++_++name, trait++_++name, method++_++name)` for each
trait method call
** Uses `method++_++name.span` as key for unique identification
* `lib/codegen.ml`:
** Added `trait++_++registry` and `trait++_++method++_++calls` to
context
** Implemented `TopImpl` monomorphization (generates mangled functions)
** Enhanced `ExprApp` handler to detect and dispatch trait method calls
** Pattern matches on `ExprField(receiver, method)` and checks hashtable
** Direct call generation to monomorphized functions
* `bin/main.ml`:
** Updated compilation pipeline to pass trait registry and call sites to
codegen
** Threads type checking context through to codegen phase

*End-to-End Flow:* 1. Type checker identifies trait method calls during
synthesis 2. Records call site info:
`(type++_++name, trait++_++name, method++_++name)` keyed by span 3.
Codegen TopImpl generates monomorphized functions with mangled names 4.
Codegen ExprApp checks if call site is a trait method 5. If yes,
generates direct Call instruction to mangled function 6. Result:
`p.show()` → `Call Point++_++Display++_++show` with `p` as first
argument
