nidula 0.2.3

A lightweight Dart library for Rust-like Option/Result types. Supports exhaustive pattern matching and compile-time safe, chainable None/Err propagation.

nidula

nidula is a lightweight library bringing Rust's Option and Result types to Dart, together with a parallel to Rust's try operator that is both compile-time safe and chainable.

nidula is a fork of option_result, which brings the following enhancements:

• Option is defined as Option<T extends Object>, thus T cannot be nullable.
• Result is defined as Result<T extends Object, E extends Object>, thus nullable types are prohibited for T and E.
  • This enforces composition with Option types (Option<T>) instead of nullable types (T?).
• Parallel to Rust's try-operator implementation rewritten from scratch.
  • Compile time safety through propagation tokens.
  • Chainable.
  • Simple library-internal error handling strategy.
  • Efficient, as no stacktraces are used for library internal propagations.
    • Propagations are errors thrown with StackTrace.empty and are already caught and handled by this library.
• Error conversion before propagation.
  • Rust's From-trait-like functionality in Dart using extensions, albeit its limitations.
• Only T v and E e fields are available.
  • value, val, err and error aliases (getters) were removed.
• There is only a single public library to import components from.
• Final modifiers to prevent extending Ok, Err, Some and None.
• == operator takes all generic types into consideration when comparing Option objects and Result objects.
• Added variable names to all function parameters in types.
  • Callback autocomplete outputs e.g. (okV) {} instead of (p0) {}.

This library aims to provide as close to a 1:1 experience in Dart as possible to Rust's implementation of these types, carrying over all of the methods for composing Option and Result values (and_then(), or_else(), map(), etc.) and allowing the use of Dart 3's new exhaustive pattern matching to provide a familiar experience while working with Option and Result type values.

Overview

Option

Option types represent the presence (Some) or absence (None) of a value.

Dart handles this pretty well on its own via null and a focus on null-safety built in to the compiler and analyzer.

The advantage of Option types over nullable types lies in their composability. Option type values have many methods that allow composing many Option-returning operations together and helpers for propagating None values in larger operations without the need for repetitive null-checking.

This supports writing clean, concise, and most importantly, safe code.

Option<int> multiplyBy5(int i) => Some(i * 5);

// NB: this is a curried function
// example: `divideBy(2)(8)` results in `Some(8 ~/ 2)`, i.e., `Some(4)`
Option<int> Function(int dividend) Function(int divisor) divideBy =
    (int divisor) => (int dividend) => switch (divisor) {
          0 => None(),
          _ => Some(dividend ~/ divisor),
        };

void main() {
  Option<int> a = Some(10);
  Option<int> b = Some(0);
  Option<int> c = None();

  Option<int> d = a.andThen(divideBy(2).call).andThen(multiplyBy5); // Some(25)
  Option<int> e = a.andThen(divideBy(0).call).andThen(multiplyBy5); // None()

  Option<int> f = b.andThen(divideBy(2).call).andThen(multiplyBy5); // Some(0)
  Option<int> g = b.andThen(divideBy(0).call).andThen(multiplyBy5); // None()

  Option<int> h = c.andThen(divideBy(2).call).andThen(multiplyBy5); // None()
  Option<int> i = c.andThen(divideBy(0).call).andThen(multiplyBy5); // None()
}

Parallel to Rust's try-operator implementation

With Option types, a parallel to Rust's try-operator is achieved combining try_ with syncTryScope (for synchronous functions) or asyncTryScope (for asynchronous ones). For those unfamiliar with Rust, the try operator tries to unwrap the Option value, however, if the Option is a None, the try operator propagates None (this way we don't have to always write code for checking for and possibly returning the None; the more potential None cases there are, the more pragmatic this pattern becomes).

// Example from Option.syncTryScope docstring
Option<int> example2(Option<int> o) {
  var l = o.map(identity); // initial `l`: a copy of `o`
  return Option.syncTryScope<int>((nt) {
    l = Some(l.try_(nt) + [1, 2, 3].elementAt(1));
    // it will propagate now if initial `l` was None, else continues
    l = None(); // not propagating yet
    l.try_(nt); // it will propagate now if initial `l` was Some
    l = Some(l.try_(nt) + [5, 6].elementAt(1)); // dead code (not detected by IDE)
    return Some(l.try_(nt));
  });
}

Option<int> myOption = example(Some(9));

switch (myOption) {
  case Some(:int v): print('Contained value: $v');
  case None(): print('None');
}

NonePropagationToken features a private constructor, thus l.try_(NonePropagationToken()) cannot be used to pass the required argument (and thus execute the method).

The provided argument nt in the syncTryScope callback is an instance of NonePropagationToken, and is expected to be passed to try_. The propagation that is thrown inside the fn argument of syncTryScope must be handled by syncTryScope's body. If there is no syncTryScope, then there cannot be any a NonePropagationToken nt, making l.try_ impossible to invoke. Therefore, the NonePropagationToken guarantees compile-time safety.

The same holds for asyncTryScope.

Note that the try_ method allows chaining, for example: return Ok(a.try_(nt).makeCall().try_(nt).makeSecondCall().try_(nt)), where makeCall and makeSecondCall must be methods defined in T returning Option<T>.

Nested options and comparison to nullable types

A big difference between Option types and nullable types is that Option types can be nested. For example: both None() and Some(None()) are valid values for Option<Option<int>>.

On the other hand, with nullable types some structures are just not possible. For example, the type int?? is not something similar to Option<Option<int>>; on the contrary, is exactly the same as int?. Thus, the distinction between None() and Some(None()) is just not possible to do with null.

Nested options are mostly useful e.g. when we do a find in a list of Options.

Result

Result types represent the result of some operation, either success (Ok), or failure (Err), and both variants can hold data.

This promotes safe handling of error values without the need for try/catch blocks while also providing composability like Option via methods for composing Result-returning operations together and helpers for propagating Err values within larger operations without the need for repetitive error catching, checking, and rethrowing.

Again, like Option, this helps promote clean, concise, and safe code.

Result<int, String> multiplyBy5(int i) => Ok(i * 5);

// NB: this is a curried function
// example: `divideBy(2)(8)` results in `Ok(8 ~/ 2)`, i.e., `Ok(4)`
Result<int, String> Function(int dividend) Function(int divisor) divideBy =
    (int divisor) => (int dividend) => switch (divisor) {
          0 => Err('divided by 0'),
          _ => Ok(dividend ~/ divisor),
        };

void main() {
  Result<int, String> a = Ok(10);
  Result<int, String> b = Ok(0);
  Result<int, String> c = Err('foo');

  Result<int, String> d = a.andThen(divideBy(2).call).andThen(multiplyBy5); // Ok(25)
  Result<int, String> e = a.andThen(divideBy(0).call).andThen(multiplyBy5); // Err(divided by 0)

  Result<int, String> f = b.andThen(divideBy(2).call).andThen(multiplyBy5); // Some(0)
  Result<int, String> g = b.andThen(divideBy(0).call).andThen(multiplyBy5); // Err(divided by 0)

  Result<int, String> h = c.andThen(divideBy(2).call).andThen(multiplyBy5); // Err(foo)
  Result<int, String> i = c.andThen(divideBy(0).call).andThen(multiplyBy5); // Err(foo)
}

Why Result's T is not nullable

This library's choice for Result's T type is opinionated: T could have been nullable (i.e. the signature of Result would have been Result<T, E extends Object). However, due to Options advantages over nullable types (such as nesting and composition helpers), it was decided to exclude all nullable types.

Additionally, weird types such as Result<Option<String>?, int> are automatically prevented because of this restriction. Also, this design prevents making a choice between nullable and Option types whenever specifying the type T where absence of value can take place; forcing the usage of Option in these cases arguably results in a more uniform style.

On request, a fork of this library supporting nullable T types for Result could be created.

Parallel to Rust's try-operator implementation

With Result types, a parallel to Rust's try-operator is achieved combining try_ with syncTryScope (for synchronous functions) or asyncTryScope (for asynchronous ones). For those unfamiliar with Rust, the try operator tries to unwrap the Result value, however, if the Result is a None, the try operator propagates Err (this way we don't have to always write code for checking for and possibly returning the Err; the more potential Err cases there are, the more pragmatic this pattern becomes).

// Example from Result.syncTryScope docstring
Result<double, String> example2(Result<double, String> r) {
  var s = r.map(identity); // initial `s`: a copy of `r`
  return Result.syncTryScope((et) {
    s = Ok(s.try_(et) / 2); // it will propagate now if initial `s` was Err
    s = Err('not propagating yet');
    s.try_(et); // it will propagate now if initial `s` was Ok
    s = Ok(s.try_(et) / 0); // dead code (not detected by IDE)
    return Ok(s.try_(et));
  });
}

Result<double, String> myResult = example2(Ok(0.9));

switch (myResult) {
  case Ok(:double v): print('Ok value: $v');
  case Err(:String e): print('Error: $e');
}

ErrPropagationToken features a private constructor, thus l.try_(ErrPropagationToken()) cannot be used to pass the required argument (and thus execute the method).

The provided argument et in the syncTryScope callback is an instance of ErrPropagationToken, and is expected to be passed to try_. The propagation that is thrown inside the fn argument of syncTryScope must be handled by syncTryScope's body. If there is no syncTryScope, then there cannot be any a ErrPropagationToken et, making l.try_ impossible to invoke. Therefore, the ErrPropagationToken guarantees compile-time safety.

The same holds for asyncTryScope.

Note that the try_ method allows chaining, for example: return Some(a.try_(et).makeCall().try_(et).makeSecondCall().try_(et)), where makeCall and makeSecondCall must be methods defined in T returning Result<T, E>.

Empty tuple

But Result doesn't always have to concern data. A Result can be used strictly for error handling, where an Ok simply means there was no error and you can safely continue. In Rust this is typically done by returning the unit type () as Result<(), E> and the same can be done in Dart with an empty Record via ().

Result<(), String> failableOperation() {
  if (someReasonToFail) {
    return Err('Failure');
  }
  return Ok(());
}

Result<(), String> err = failableOperation();

if (err case Err(e: String error)) {
  print(error);
  return;
}

// No error, continue...

To further support this, just like how you can unwrap Option and Result values by calling them like a function, an extension for Future<Option<T>> and Future<Result<T, E>> is provided to allow calling them like a function as well which will transform the future into a future that unwraps the resulting Option or Result when completing.

(This also applies to FutureOr values.)

// Here we have two functions that return Result<(), String>, one of which is a Future.
// We can wrap them in a asyncTryScope block (async in this case) and call them like a function
// to unwrap them, discarding the unit value if Ok, or propagating the Err value otherwise.
Result<(), String> err = await Result.asyncTryScope((et) async {
  (await failableOperation1()).try_(et);
  failableOperation2().try_(et);

  return Ok(());
});

if (err case Err(e: String error)) {
  print(error);
  return;
}

// No error, continue...

Note that just like how unit has one value in Rust, empty Record values in Dart are optimized to the same runtime constant reference so there is no performance or memory overhead when using () as a unit type.

Try-catch warning

Using try catch in combination with Result.(a)syncTryScope or Option.(a)syncTryScope can be done, however we need to ensure NonePropagation and ErrPropagation are handled only inside Result.(a)syncTryScope/Option.(a)syncTryScope.

If the try block wraps the syncTryScope/asyncTryScope function and there is no outer syncTryScope/asyncTryScope wrapping the try-catch block, then it is fine. For example:

Result<double, String> example3(Result<double, String> r) {
  var s = r.map(identity); // initial `s`: a copy of `r`
  try {
    return Result.syncTryScope((et) {
      s = Ok(s.try_(et) / 2); // it will propagate now if initial `s` was Err
      throw 'example';
      s = Err('not propagating yet'); // dead code
      s.try_(et);
      s = Ok(s.try_(et) / 0);
      return Ok(s.try_(et));
    });
  } on String {
    return Err('caught a String');
  }
}

However, we must be a little careful with a try-catch inside the syncTryScope/asyncTryScope's callback or any function that is called inside of it.

Bad example

The next example catches also ErrPropagation<String> (that is thrown by try_ if s is an Err), which compromises the error propagation.

Result<double, String> badExample(Result<double, String> r) {
  var s = r.map(identity);
  return Result.syncTryScope<double, String>((et) {
    try {
      s = Ok(s.try_(et) / [1,2,3].elementAt(100));
    } catch (e) {
      s = Err('index too high');
    }
    return Ok(s.try_(et));
  });
}

Good — Catch specific errors if possible

Catching the exact exceptions/errors that might be thrown — thus, avoiding catching all possible errors with } on catch (e) { — would be the ideal approach:

Result<double, String> goodExample1(Result<double, String> r) {
  var s = r.map(identity);
  return Result.syncTryScope<double, String>((et) {
    try {
      s = Ok(s.try_(et) / [1,2,3].elementAt(100));
    } on RangeError catch (e) {
      s = Err('index too high');
    }
    return Ok(s.try_(et));
  });
}

Good — When catching specific errors is not possible

If it is not possible to catch the exact errors, or there would be too many to distinguish from, then always rethrow Propagation:

Result<double, String> goodExample2(Result<double, String> r) {
  var s = r.map(identity);
  return Result.syncTryScope<double, String>((et) {
    try {
      s = Ok(s.try_(et) / [1,2,3].elementAt(100));
    } on Propagation {
      rethrow; // always rethrow so that the contained error propagates
    } catch (e) {
      s = Err('index too high');
    }
    return Ok(s.try_(et));
  });
}

Error conversion before propagation

In Rust, it is possible to covert errors leveraging the From trait. With Dart, there is not an equivalent.

The first solution is to use mapErr every time. This might however inflate the verbosity of the code.

In order to get a solution that is close to the From trait, we can define extensions in our projects and define the conversions there. For each conversion we need to define a new method, e.g., tryCvtBool, which converts the error contained in Err<(), int> (which is of int type) to a bool before propagating it.

For example:

// ignore_for_file: unused_local_variable

import 'package:nidula/nidula.dart';

extension ResultIntErr<T extends Object> on Result<T, int> {
  T tryCvtString(ErrPropagationToken<String> et) {
    return mapErr((errE) => '$errE').try_(et);
  }

  T tryCvtBool(ErrPropagationToken<bool> et) {
    return mapErr((errE) => errE == 1).try_(et);
  }
}

extension ResultBoolErr<T extends Object> on Result<T, bool> {
  T tryCvtString(ErrPropagationToken<String> et) {
    return mapErr((errE) => '$errE').try_(et);
  }
}

void a() {
  final Result<(), int> ok = Ok(());
  final a = Result.syncTryScope<(), int>((et) {
    return Ok(ok.try_(et));
  });
  final b = Result.syncTryScope<(), String>((et) {
    return Ok(ok.tryCvtString(et));
  });
  final c = Result.syncTryScope<(), bool>((et) {
    return Ok(ok.tryCvtBool(et));
  });

  final Result<(), bool> ok2 = Ok(());
  final a2 = Result.syncTryScope<(), bool>((et) {
    return Ok(ok2.try_(et));
  });
  final b2 = Result.syncTryScope<(), String>((et) {
    return Ok(ok2.tryCvtString(et));
  });
}

Key differences from Rust

• Option and Result types provided by this library are immutable. All composition methods either return new instances or the same instance unmodified if applicable, and methods for inserting/replacing values are not provided.
  
  
• This library lacks all of the methods Rust's Option and Result types have that are related to ref, deref, mut, pin, clone, and copy due to not being applicable to Dart as a higher-level language.
  
  
• The Option.filter() method has been renamed where() to be more Dart-idiomatic.
  
  
• The Option and Result methods mapOr, mapOrElse return Option<U> and Result<U, E> respectively to aid composition of Option and Result values. The encapsulated values of these types should never leave the context of Option or Result unless explicitly unwrapped via the designated methods (unwrap(), expect(), etc.).
  
  
• None()/Err() propagation is not supported at the language level in Dart since there's no concept of it so it's not quite as ergonomic as Rust, but is still quite comfy and easily managed via the provided helpers.