Result & Option

Result and Option are Tova's primary error-handling types. Instead of throwing exceptions, Tova functions return Result or Option values that explicitly represent success/failure or presence/absence.

This is the most important part of the standard library. Master these types and you will write robust, predictable Tova code.

Overview

Type	Variants	Use When
Result	Ok(value), Err(error)	An operation can succeed or fail with an error
Option	Some(value), None	A value may or may not exist

// Result: parsing can fail
fn parse_int(s) {
  n = Int.new(s)
  if n != n {    // NaN check
    Err("not a valid integer: {s}")
  } else {
    Ok(floor(n))
  }
}

// Option: finding might return nothing
fn find_user(id) {
  user = db.get(id)
  if user == nil { None } else { Some(user) }
}

---

Result Type

The Result type represents the outcome of an operation that can succeed or fail.

Creating Results

success = Ok(42)
failure = Err("something went wrong")

Ok({ name: "Alice", age: 30 })
Err({ code: 404, message: "not found" })

Ok Methods

When a Result is Ok(value), these methods behave as follows:

.map(fn)

Transforms the success value. Returns Ok(fn(value)).

Ok(5).map(fn(x) x * 2)          // Ok(10)
Ok("hello").map(upper)           // Ok("HELLO")

.flatMap(fn)

Chains operations that themselves return Results. The function must return an Ok or Err.

Ok(5).flatMap(fn(x) {
  if x > 0 { Ok(x * 2) } else { Err("must be positive") }
})
// Ok(10)

Ok(-1).flatMap(fn(x) {
  if x > 0 { Ok(x * 2) } else { Err("must be positive") }
})
// Err("must be positive")

.unwrap()

Extracts the value from Ok. Throws if called on Err.

Ok(42).unwrap()                  // 42
// Err("fail").unwrap()          -- throws!

.unwrapOr(default)

Extracts the value, or returns the default if this is an Err.

Ok(42).unwrapOr(0)               // 42
Err("fail").unwrapOr(0)          // 0

.expect(msg)

Like .unwrap() but throws with a custom message on Err.

Ok(42).expect("should have value")   // 42
// Err("x").expect("oh no")          -- throws "oh no"

.isOk() / .isErr()

Ok(42).isOk()                    // true
Ok(42).isErr()                   // false

.mapErr(fn)

Transforms the error value. On Ok, returns self unchanged.

Ok(42).mapErr(fn(e) "wrapped: {e}")   // Ok(42)  -- no change

.unwrapErr()

Extracts the error value. Throws if called on Ok.

// Ok(42).unwrapErr()            -- throws!

.or(other)

Returns self (Ok) when the result is Ok.

Ok(42).or(Ok(99))                // Ok(42)

.and(other)

Returns other when the result is Ok.

Ok(42).and(Ok(99))               // Ok(99)
Ok(42).and(Err("fail"))          // Err("fail")

---

Err Methods

When a Result is Err(error), these methods behave as follows:

.map(fn)

Returns self (Err) unchanged. The function is not called.

Err("fail").map(fn(x) x * 2)    // Err("fail")

.flatMap(fn)

Returns self (Err) unchanged. The function is not called.

Err("fail").flatMap(fn(x) Ok(x * 2))   // Err("fail")

.unwrap()

Throws with the error value.

// Err("something broke").unwrap()  -- throws "something broke"

.unwrapOr(default)

Returns the default value.

Err("fail").unwrapOr(0)          // 0
Err("fail").unwrapOr("fallback") // "fallback"

.expect(msg)

Throws with the custom message.

// Err("x").expect("config missing")  -- throws "config missing"

.isOk() / .isErr()

Err("fail").isOk()               // false
Err("fail").isErr()              // true

.mapErr(fn)

Transforms the error value. Returns Err(fn(error)).

Err("not found").mapErr(fn(e) "Error: {e}")
// Err("Error: not found")

.unwrapErr()

Extracts the error value.

Err("fail").unwrapErr()          // "fail"

.or(other)

Returns other since this result is an Err.

Err("fail").or(Ok(99))           // Ok(99)
Err("a").or(Err("b"))            // Err("b")

.and(other)

Returns self (Err) since the first result already failed.

Err("fail").and(Ok(99))          // Err("fail")

---

Complete Result Method Reference

Method	On Ok(v)	On Err(e)
.map(fn)	Ok(fn(v))	Err(e)
.flatMap(fn)	fn(v) (must return Result)	Err(e)
.unwrap()	v	throws e
.unwrapOr(def)	v	def
.expect(msg)	v	throws msg
.isOk()	true	false
.isErr()	false	true
.mapErr(fn)	Ok(v)	Err(fn(e))
.unwrapErr()	throws	e
.or(other)	Ok(v)	other
.and(other)	other	Err(e)

---

Option Type

The Option type represents a value that may or may not exist. Use it instead of nil checks.

Creating Options

present = Some(42)
absent = None

None is a singleton -- there is only one None value.

Some Methods

When an Option is Some(value):

.map(fn)

Transforms the inner value. Returns Some(fn(value)).

Some(5).map(fn(x) x * 2)        // Some(10)
Some("hello").map(upper)         // Some("HELLO")

.flatMap(fn)

Chains operations that return Options. The function must return Some or None.

Some(5).flatMap(fn(x) {
  if x > 0 { Some(x * 2) } else { None }
})
// Some(10)

.unwrap()

Extracts the value. Throws if called on None.

Some(42).unwrap()                // 42

.unwrapOr(default)

Extracts the value, or returns the default.

Some(42).unwrapOr(0)             // 42

.expect(msg)

Like .unwrap() but throws with a custom message on None.

Some(42).expect("need value")    // 42

.isSome() / .isNone()

Some(42).isSome()                // true
Some(42).isNone()                // false

.or(other)

Returns self (Some).

Some(42).or(Some(99))            // Some(42)

.and(other)

Returns other when this Option has a value.

Some(42).and(Some(99))           // Some(99)
Some(42).and(None)               // None

.filter(pred)

Returns Some(value) if the predicate returns true, otherwise None.

Some(5).filter(fn(x) x > 3)     // Some(5)
Some(1).filter(fn(x) x > 3)     // None

---

None Methods

When an Option is None:

Method	Behavior
.map(fn)	None
.flatMap(fn)	None
.unwrap()	throws
.unwrapOr(def)	def
.expect(msg)	throws msg
.isSome()	false
.isNone()	true
.or(other)	other
.and(other)	None
.filter(pred)	None

None.map(fn(x) x * 2)           // None
None.unwrapOr(0)                 // 0
None.or(Some(99))                // Some(99)
None.filter(fn(x) true)         // None

---

Complete Option Method Reference

Method	On Some(v)	On None
.map(fn)	Some(fn(v))	None
.flatMap(fn)	fn(v) (must return Option)	None
.unwrap()	v	throws
.unwrapOr(def)	v	def
.expect(msg)	v	throws msg
.isSome()	true	false
.isNone()	false	true
.or(other)	Some(v)	other
.and(other)	other	None
.filter(pred)	if pred(v) { Some(v) } else { None }	None

---

Propagation Operator ?

The ? operator provides concise syntax for unwrapping Ok/Some values and short-circuiting on Err/None. It works like Rust's ? operator.

When applied to a value:

• If the value is Ok(v) or Some(v), it extracts v and execution continues
• If the value is Err(e) or None, the enclosing function immediately returns that Err or None

fn process(input) {
  // Without propagation
  result = parse(input)
  if result.isErr() {
    return result
  }
  value = result.unwrap()

  // With propagation -- equivalent to above
  value = parse(input)?
}

Example: Chaining Fallible Operations

fn process_user(raw_data) {
  // Each ? unwraps Ok or short-circuits with Err
  parsed = parse_json(raw_data)?
  validated = validate_user(parsed)?
  saved = save_to_db(validated)?
  Ok(saved)
}

// Calling it:
match process_user(data) {
  Ok(user) => print("Saved: {user.name}")
  Err(e) => print("Failed: {e}")
}

How It Works Internally

The propagation operator uses a sentinel-based mechanism:

• __propagate(val) -- if the value is Err or None, throws an internal sentinel object; if Ok or Some, returns the inner value
• The enclosing function catches the sentinel and returns the Err/None directly

You never need to call __propagate manually -- just use the ? suffix operator.

---

Practical Patterns

Chaining with .map() and .flatMap()

Use .map() when the transformation cannot fail. Use .flatMap() when the transformation itself returns a Result or Option.

// .map() -- transform cannot fail
Ok(5)
  .map(fn(x) x * 2)
  .map(fn(x) x + 1)
// Ok(11)

// .flatMap() -- each step can fail
fn parse_and_double(s) {
  parse_int(s)
    .flatMap(fn(n) {
      if n > 1000 {
        Err("number too large")
      } else {
        Ok(n * 2)
      }
    })
    .map(fn(n) "Result: {n}")
}

parse_and_double("42")       // Ok("Result: 84")
parse_and_double("abc")      // Err("not a valid integer: abc")
parse_and_double("9999")     // Err("number too large")

Providing Defaults

// Use .unwrapOr() for a default value
config_port = get_env("PORT")
  .map(fn(s) parse_int(s).unwrapOr(3000))
  .unwrapOr(3000)

// Use .or() to try alternatives
fn find_config() {
  load_file("config.local.json")
    .or(load_file("config.json"))
    .or(Ok(default_config))
}

Pattern Matching on Results and Options

match fetch_user(id) {
  Ok(user) => render_profile(user)
  Err("not found") => render_404()
  Err(e) => render_error(e)
}

match find(items, fn(x) x.id == target_id) {
  Some(item) => print("Found: {item.name}")
  None => print("Not found")
}

Converting Between Result and Option

// Result to value-or-nil
value = result.unwrapOr(nil)

// Check and branch
if result.isOk() {
  handle_success(result.unwrap())
} else {
  handle_error(result.unwrapErr())
}

Error Transformation

// Wrap low-level errors with context
fn load_user_config(path) {
  read_file(path)
    .mapErr(fn(e) "failed to read config at {path}: {e}")
    .flatMap(fn(content) {
      parse_json(content)
        .mapErr(fn(e) "invalid JSON in config: {e}")
    })
}

When to Use Result vs Option

Use Result when:

• An operation can fail and the caller needs to know why
• You need to propagate error messages or codes
• The failure represents an error condition

// Result: the caller needs to know what went wrong
fn divide(a, b) {
  if b == 0 {
    Err("division by zero")
  } else {
    Ok(a / b)
  }
}

Use Option when:

• A value might simply not exist
• There is no meaningful error to report
• You are doing lookups or searches

// Option: the item might not exist, and that is fine
fn find_by_name(users, name) {
  result = find(users, fn(u) u.name == name)
  if result == nil { None } else { Some(result) }
}

---

Collection Helpers

filter_ok

filter_ok(results) -> [T]

Filters an array of Result values, returning only the unwrapped Ok values and discarding any Err entries.

results = [Ok(1), Err("bad"), Ok(2), Err("fail"), Ok(3)]
filter_ok(results)    // [1, 2, 3]

filter_err

filter_err(results) -> [E]

Filters an array of Result values, returning only the unwrapped Err values and discarding any Ok entries.

results = [Ok(1), Err("bad"), Ok(2), Err("fail"), Ok(3)]
filter_err(results)    // ["bad", "fail"]

These are useful when processing batches of results:

results = urls |> map(fn(url) try_fn(fn() fetch(url)))
successes = filter_ok(results)
failures = filter_err(results)
print("{len(successes)} succeeded, {len(failures)} failed")