# Gotlin

`Gotlin` is a small Kotlin-like frontend implemented in Go that targets the Go toolchain.

This is the practical boundary of the prototype:

- It is a Kotlin-flavored language frontend.
- It targets the Go toolchain by generating valid Go source and building through `go build`.
- It is not a direct integration into Go's internal `cmd/compile` backend APIs.

The compiler keeps source spelling in its syntax AST, then builds lexical
symbols, structural semantic types, resolved expression meanings, and typed
HIR before Go emission. Class reference semantics live in `ClassType`; only
the semantic Type-to-Go mapping turns a class such as `User` into `*User`.

## Supported language slice

- `fun` declarations
- `val` and `var`
- `Int`, `Long`, `String`, `Boolean`, `Unit`
- function types like `(String) -> Unit`
- `if`, `else`, `while`, and `for (item in items)`
- function calls
- lambdas like `{ x: Int -> println(x) }` and `{ println(it) }`
- `class` with primary-constructor fields and methods
- `interface` with method signatures
- Rust-style algebraic `enum` declarations with payload variants and exhaustive `match`
- `println(...)`
- arithmetic, comparison, and boolean operators
- decimal literals, `defer`, and Go address-of expressions such as `&value`
- explicit nullable types (`Type?`), safe access (`?.`), and non-null assertions (`!!`)
- Gotlin classes are reference types by default; `*` is only needed for external Go pointer types
- named external Go struct construction, for example `http.Client(timeout = 3 * time.second)`
- top-level embedded resources such as `@embed("assets/*") val assets: embed.FS`
- structured coroutines with `suspend fun`, `runBlocking`, `coroutineScope`, `launch`, `async`, `await`, `delay`, `withTimeout`, and `isActive`

## Example

```kotlin
package demo

fun fib(n: Int): Int {
    if (n < 2) {
        return n
    }
    return fib(n - 1) + fib(n - 2)
}

fun main() {
    println(fib(8))
}
```

Imports from Go packages are supported:

```kotlin
package demo

import strings

fun main() {
    println(strings.ToUpper("gotlin"))
}
```

HTTP server example:

```kotlin
package demo.web

import fmt
import net.http

fun helloHandler(w: http.ResponseWriter, r: *http.Request) {
    fmt.Fprintln(w, "hello from gotlin")
}

fun main() {
    http.HandleFunc("/", helloHandler)
    fmt.Println("serving http://localhost:8080")
    http.ListenAndServe(":8080", http.DefaultServeMux)
}
```

Classes and interfaces:

```kotlin
package demo

interface Greeter {
    fun greet(name: String): String
}

class ConsoleGreeter(val prefix: String) {
    fun greet(name: String): String {
        return prefix + name
    }
}

fun main() {
    val greeter: Greeter = ConsoleGreeter("hello, ")
    println(greeter.greet("gotlin"))
}
```

Rust-style enums:

```kotlin
enum PaymentResult {
    Accepted(String)
    Rejected(String)
    Pending
}

fun describe(result: PaymentResult): String {
    return match (result) {
        PaymentResult.Accepted(id) -> id
        PaymentResult.Rejected(reason) -> reason
        PaymentResult.Pending -> "pending"
    }
}
```

Enum matches must contain each variant exactly once. Variant payload arity is
checked during Gotlin compilation. A match used as an expression also requires
every arm to return the same type. Block-style statement matches remain
available for side effects.

## Null safety

Types are non-nullable by default. Add `?` explicitly when `null` is valid:

```kotlin
fun email(user: User?): String? {
    return user?.email
}
```

The compiler rejects `null` in non-nullable arguments, fields, local variables,
and return values. Nullable receivers cannot be dereferenced directly. Gotlin
smart-casts values after `value != null` branches and guard clauses such as
`if (value == null) { return }`. Use `!!` only when an invariant cannot be
expressed through control flow:

```kotlin
val required: User = optionalUser!!
```

## Error handling

Gotlin adapts Go `(T, error)` returns into Rust-style `Result<T, Error>` flows.
An explicit `Result` return or variable type converts the Go return directly:

```kotlin
fun parse(value: String): Result<Int, Error> {
    return strconv.atoi(value)
}

fun ping(db: *sql.DB): Result<Unit, Error> {
    return db.ping()
}
```

Use `?` to propagate a Go error or chain a Gotlin function returning `Result`.
Explicit panic and fallback operations are available when appropriate:

```kotlin
val required = parse("42").unwrap()
val fallback = parse("invalid").unwrapOr(0)
```

Gotlin never inserts implicit panic wrappers. A Go call returning `(T, error)`
must have an explicit `Result` context or use `?`, explicit `value, error`
destructuring, or `.unwrap()`.

Gotlin-defined `class` and `data class` values are references automatically,
including nested generic types such as `List<User>`. Explicit pointer syntax is
reserved for Go interop, for example `*http.Request` and `*pgxpool.Pool`;
applying `*` to a Gotlin class is a compile error.

Enums whose variants carry no payload are represented as string-backed values.
The exact variant identifier is used for JSON and PostgreSQL text values:

```kotlin
enum PaymentStatus {
    PENDING_RESERVATION
    INITIATED
}
```

This reads and writes `"PENDING_RESERVATION"` and `"INITIATED"` directly.
Payload-carrying enums remain algebraic sum types.

## Structural mapping

Compatible classes and enums can be converted with `mapTo<T>()`:

```kotlin
data class AddressEntity(var city: String)
data class AddressResponse(var city: String)
data class AccountEntity(var id: String, var address: *AddressEntity)
data class AccountResponse(var address: *AddressResponse, var id: String)

val response = account.mapTo<AccountResponse>()
```

When the surrounding expression provides a target type, the type argument is
optional:

```kotlin
fun response(account: AccountEntity): AccountResponse {
    return account.mapTo()
}

val response: AccountResponse = account.mapTo()
val envelope = Envelope(account.mapTo())
```

Use explicit `mapTo<T>()` when assigning to an untyped local or when no target
type can be inferred.

Fields are matched by Gotlin name rather than declaration order. Mapping is
recursive across nested classes, pointers, nullable values, lists, mutable
lists, maps, and enum payloads. Enum variants are matched by name. Extra source
fields and extra target enum variants are allowed; every target field and every
source enum variant must be compatible.

Payloadless enums also map recursively to and from `String`. String-to-enum
mapping validates the runtime value and panics for an unknown variant string.

Incompatible mappings fail compilation with a complete path, for example:

```text
cannot map Account.address.zip: String is incompatible with Int
```

```bash
go run ./cmd/gotlinc build ./examples/hello.gt
./hello
```

Emit Go source instead:

```bash
go run ./cmd/gotlinc build -src ./examples/hello.gt -o /tmp/hello.go
go run /tmp/hello.go
```

Run directly:

```bash
go run ./cmd/gotlinc run ./examples/hello.gt
```

## Structured coroutines

Gotlin coroutines use Go goroutines underneath, but expose only structured
scopes. A scope waits for its children, propagates child failures, and cancels
sibling coroutine contexts. The removed `worker`, bare `go`, and channel
`select` forms are not valid Gotlin syntax; use coroutine scopes, `delay`, and
explicit channel `read()`/`send()` operations.

```kotlin
suspend fun load(): Int {
    delay(10)
    return 42
}

fun main() {
    runBlocking {
        val value = async<Int> { return load() }
        launch { println("loading") }
        println(value.await())
    }
}
```

## Type-checked SQL queries

Gotlin recognizes a PostgreSQL SQL DSL at compile time. SQL row mappings must be data classes annotated with `@table`. Fields map from lower-camel Gotlin names to `snake_case` columns by default and can override the SQL name with `@column`. Conflict keys use `@id`; database-generated or defaulted fields use `@generated`.

```kotlin
import time

@table("accounts")
data class AccountRow(
    @generated @id var id: String,
    var customerId: String,
    @column("kind") var accountType: String,
    var balance: Double,
    var closedAt: time.Time?
)

fun accountsFor(customerId: String): GotlinSQLQuery {
    return sql.from<AccountRow>()
        .where { row -> row.customerId == customerId && row.closedAt == null }
        .orderByDescending { row -> row.balance }
        .limit(100)
        .build()
}
```

The compiler emits a Go value with this generated support type:

```go
type GotlinSQLQuery struct {
    SQL  string
    Args []any
}
```

The example selects every field, including `@generated` fields, and produces PostgreSQL `$n` placeholders. Arguments are emitted in SQL traversal order. A literal `limit(100)` is embedded after validation; a typed non-literal `Int` limit uses the next placeholder.

Nullable types use a `?` suffix, currently including forms such as `String?` and `time.Time?`; generated Go fields use pointers. Comparing a nullable field with `null` lowers to `IS NULL` or `IS NOT NULL`. Ordering comparisons support numeric values and `time.Time`; typed `now()` emits `CURRENT_TIMESTAMP` without an argument:

```kotlin
sql.from<OutboxRow>()
    .where { it.publishedAt == null && (it.claimedUntil == null || it.claimedUntil < now()) }
    .orderBy { it.createdAt }
    .limit(batchSize)
    .forUpdate()
    .skipLocked()
    .build()
```

The canonical select method order is:

```text
sql.from<Row>()
    [.select { ... }]
    [.where { ... }]
    [.orderBy { ... } | .orderByDescending { ... }]
    [.limit(Int)]
    [.forUpdate()]
    [.skipLocked()]
    .build() | .fetch(pool, ctx) | .single(pool, ctx) | .iterator(pool, ctx)
```

Each optional method may occur at most once. `select` must be first, `skipLocked` requires `forUpdate`, and clauses are emitted as `WHERE`, `ORDER BY`, `LIMIT`, `FOR UPDATE`, `SKIP LOCKED` in PostgreSQL order.

### Typed projections

Projection targets are local data classes. The constructor must contain one direct source-row field per target field, in target declaration order, with exact matching types:

```kotlin
data class AccountSummary(var id: String, var balance: Double)

fun summaries(pool: *pgxpool.Pool, ctx: context.Context): List<AccountSummary> {
    return sql.from<AccountRow>()
        .select { row -> AccountSummary(row.id, row.balance) }
        .orderBy { it.balance }
        .fetch(pool, ctx)
        .unwrap()
}
```

The compiler emits `SELECT id, balance`, scans in projection declaration order, and makes `fetch`, `single`, and `iterator` target `AccountSummary` rather than `AccountRow`. The row parameter in later `where` and ordering methods still represents `AccountRow`.

Select chains can execute directly against a pgx/v5 pool. `fetch(pool, ctx)`
returns `Result<List<AccountRow>, Error>` and closes pgx rows after reading and
scanning every result in data-class field declaration order:

```kotlin
import context
import pgxpool "github.com/jackc/pgx/v5/pgxpool"

fun accountsFor(
    pool: *pgxpool.Pool,
    ctx: context.Context,
    customerId: String
): List<AccountRow> {
    return sql.from<AccountRow>()
        .where { it.customerId == customerId }
        .orderBy { it.accountType }
        .fetch(pool, ctx)
        .unwrap()
}
```

`single(pool, ctx)` returns `Result<AccountRow, Error>` and closes the rows. Use
`?` to propagate zero/multiple-row errors or call `.unwrap()` explicitly:

```kotlin
fun account(pool: *pgxpool.Pool, ctx: context.Context, id: String): AccountRow {
    return sql.from<AccountRow>()
        .where { it.id == id }
        .single(pool, ctx)
        .unwrap()
}
```

`iterator(pool, ctx)` returns a `Result` around a streaming pgx iterator. Call
`next()` before each `value()`, arrange an explicit `close()`, and inspect
`err()` after iteration:

```kotlin
fun printAccounts(pool: *pgxpool.Pool, ctx: context.Context) {
    val rows = sql.from<AccountRow>().iterator(pool, ctx).unwrap()
    defer rows.close()

    while (rows.next()) {
        val account: AccountRow = rows.value()
        println(account.customerId)
    }

    val checked = rows.err()
}
```

Query and scan failures are returned as `Result` errors. `fetch` and `single`
close rows internally; an iterator leaves lifecycle control with the caller,
and `value()` panics only when called without a successful `next()`. None of the
execution terminals require an intermediate `build()`, `query.sql`, or
`query.args` access. `build()` remains available for manual execution.

### Inserts and returning

Inserts omit every `@generated` field. If all fields are generated, the compiler emits `INSERT ... DEFAULT VALUES`. Conflict handling is optional; conflict fields must have `@id`:

```kotlin
sql.insert<AccountRow>(row)
    .onConflict { it.id }
    .doNothing()
    .build()
```

Composite conflict keys use `listOf`, for example `.onConflict { listOf(it.tenantId, it.id) }`.

Updates use a typed lower-camel `set` form because Gotlin does not currently implement Kotlin callable references (`AccountRow::balance`):

```kotlin
sql.insert<AccountRow>(row)
    .onConflict { it.id }
    .doUpdate { excluded -> set(AccountRow.balance, excluded.balance) }
    .build()
```

Multiple `set(...)` expressions may appear in one `doUpdate` lambda. Update targets must exist and be `var`; excluded fields must exist and have a compatible type.

An insert can return the full row or a typed projection. Write chains can use `fetch`, `single`, or `iterator` only after `returning`:

```kotlin
sql.insert<AccountRow>(row)
    .returning { it }
    .single(pool, ctx)

sql.insert<AccountRow>(row)
    .onConflict { it.id }
    .doNothing()
    .returning { value -> AccountSummary(value.id, value.balance) }
    .single(pool, ctx)
```

The exact insert forms are:

```text
sql.insert<Row>(row)
    [.onConflict { field | listOf(fields...) }.doNothing() | .doUpdate { ... }]
    [.returning { it | Projection(it.field, ...) }]
    .build() | returning execution terminal
```

### Updates and deletes

Typed updates use one `set` lambda. Targets are mutable source-row fields. Values may be typed names/selectors, literals, `null` for nullable targets, `now()` for timestamps, source-row fields, or numeric `+` and `-` expressions:

```kotlin
sql.update<AccountRow>()
    .set { row ->
        set(row.balance, row.balance + amount)
        set(row.closedAt, now())
    }
    .where { it.id == accountId }
    .returning { row -> AccountSummary(row.id, row.balance) }
    .single(pool, ctx)
```

Deletes support a typed predicate and the same returning forms:

```kotlin
sql.delete<AccountRow>()
    .where { it.id == accountId }
    .returning { it }
    .single(pool, ctx)
```

The exact write forms are:

```text
sql.update<Row>()
    .set { row -> set(row.field, value); ... }
    [.where { predicate }]
    [.returning { it | Projection(it.field, ...) }]
    .build() | returning execution terminal

sql.delete<Row>()
    [.where { predicate }]
    [.returning { it | Projection(it.field, ...) }]
    .build() | returning execution terminal
```

`where` is intentionally optional for updates and deletes, so omitting it affects the whole table. There is no write `execute` terminal yet; use `build()` for manual `Exec`, or add `returning` and use a query execution terminal.

At compile time the DSL checks that the generic row type exists, is a data class with `@table`, has unique mapped columns, and contains every referenced field. It checks known predicate operand types, nullable operations, insert row types, `@id` conflict metadata, update mutability and value compatibility, method cardinality/order, lock dependencies, and projection field counts/types.

Current SQL limitations:

- Predicates support `&&`, `||`, `!`, `==`, `!=`, and numeric/timestamp `<`, `<=`, `>`, `>=`. Values must have a compiler-known type; external values become `$n` arguments.
- Projections and `returning` do not support scalar results, aliases, computed expressions, aggregates, external structs, or reordered/coerced target types. They accept a full row or direct fields passed to a local data-class constructor.
- Standalone update values do not support SQL functions other than `now()`, string expressions, intervals, casts, subqueries, or arbitrary SQL fragments. Upsert `doUpdate` values still come only from the `excluded` row.
- Inserts emit one row at a time. Bulk/multi-row inserts are not implemented.
- Table and column annotations are validated unquoted SQL identifiers. Joins, aliases, grouping, aggregates, `OFFSET`, lock strengths other than `FOR UPDATE`, and conflict predicates are not implemented.
- The compiler does not perform migrations, schema generation, database connections, or schema introspection.

Language server:

```bash
go build -o ./bin/gotlin-lsp ./cmd/gotlin-lsp
./bin/gotlin-lsp
```

VS Code extension:

```bash
cd ./tools/vscode-gotlin
npm install
npm run build
```

## Notes

- `val` is immutable after initialization; `var` can be reassigned.
- Type inference is local to declarations without an explicit type.
- Top-level declarations currently support functions, classes, and interfaces.
- `gotlinc build` produces an executable by default. If `-o` is omitted, the output name is derived from the input file name.
- `gotlinc build -src` emits Go source instead of a binary.
- `gotlinc` supports `build` and `run`, and defaults to `build` if no subcommand is given.
- Gotlin source files use the `.gt` extension.
- `gotlin-lsp` provides diagnostics, hover, and go-to-definition over stdio.
- `gotlin-lsp` can optionally use `gopls` for hover and definition on Go-imported symbols.
- the VS Code extension adds syntax highlighting, snippets, and launches the LSP for `.gt` files.