vsavkin/fpdart

Functional Programming in Dart

FP in Dart

The DartFP library provides a lot of functional programming helpers.

Installation

Add the DartFP dependency to your project’s pubspec.yaml.

name: my_project
dependencies:
  fp: '>=0.4.2 <0.5.0'

Then, run pub install.

Finally, import the DartFP libraries.

import 'package:fp/fp.dart' as _;

Common

This set of helpers wraps the Iterable methods into functions.

For example:

final printIterable = _.forEach(print);
printIterable([1,2,3]); // prints 1 2 3

Another example:

final sum = _.reduce((a,b) => a + b);
sum([1,2,3]); // returns 6

All helpers:

forEach, map, reduce, fold, any, contains, elementAt, getByKey, where, firstWhere, lastWhere, skip

invoke/getField/setField

This group of helpers allows you to call a function on a collection of objects.

class Obj {
  var field;
  noArguments() => "no arguments";
  withPosArguments(posArg) => posArg;
  withNamedArguments({key}) => key;
}

var obj = new Ojb();

_.invoke(#noArguments)(obj); // returns "noArguments"
_.invoke(#withPosArguments, ["pos"])(obj); // returns "pos"
_.invoke(#withNamedArguments, [], {#key: "value"})(obj); // returns "value"

obj.field = 'value';
_.getField(#field)(obj); // returns "value"

_.setField(#field, "new")(obj); // sets the field to "new"

You can pass strings instead of symbols.

Compose

Composes two or more functions.

var compose = _.compose([_.getField(#length), _.invoke(#trim)]);
compose("   abc  ") => returns 3

Curry

This is a very powerful helper that transforms a function taking multiple arguments into a chain of functions taking zero or more arguments.

func(pos1, pos2) => "$pos1 $pos2";
final c = _.curry(func);

expect(c("pos1", "pos2"), equals("pos1 pos2"));
expect(c("pos1")("pos2"), equals("pos1 pos2"));
expect(c()("pos1")()("pos2"), equals("pos1 pos2"));

You can think about it as follows: The function c remembers all the given arguments. When it gets all the needed arguments, the original function will be called.

Obviously, there is no real remembering happening, which is demonstrated here:

func(pos1, pos2, pos3) => "$pos1 $pos2 $pos3";
final partial = _.curry(func)("pos1");

expect(partial("pos2")("pos3"), equals("pos1 pos2 pos3"));
expect(partial("pos4")("pos5"), equals("pos1 pos4 pos5"));

The curry helper also supports optional parameters:

func(pos1, [pos2='pos2']) => "$pos1 $pos2";
final c = _.curry(func);

expect(c("pos1"), equals("pos1 pos2"));
expect(c("pos1", "pos3"), equals("pos1 pos3"));

And named parameters:

func(pos1, {named: 'named'}) => "$pos1 $named";
final c = _.curry(func);

expect(c("pos1"), equals("pos1 named"));
expect(c("pos1", named: "named2"), equals("pos1 named2"));

Named parameters can be applied in any order:

expect(c(named: "named2")("pos1"), equals("pos1 named2"));

Partial

Partially applies a function.

func(pos1, pos2, {named}) => "$pos1 $pos2 $named";

var partial = _.partial(func, ["pos1"]);
partial("pos2"); // returns "pos1 pos2";

var partial = _.partial(func, [], {"named" : "named1"});
partial("pos1", "pos2"); // returns "pos1 pos2 named1";

Misc Iterable Utils

The compact helper filters out all nulls.

_.compact([1,2,null,3]); // returns [1,2,3]

The flatten helper:

_.flatten([[1,2], [3]]); // returns [1,2,3]

The flatMap helper calls map and flattens the result:

final dup = _.flatMap((n) => [n,n]);
dup([1,2]); // returns [1,1,2,2]

The range helper generates an iterable of integers:

_.range(1,3); // returns [1,2,3]

The first helper returns the first element of the given iterable:

_.first([1,2,3]); // returns 1

The rest helper skips the first element:

_.rest([1,2,3]); // returns [2,3]

The zip zips two iterables into one:

_.zip([1,2],[3,4,5]); // returns [[1,3], [2,4]]

The groupBy helper groups all items in the iterable using the provided function.

groupBy([1,2,3,4], isEven); //{true: [2,4], false: [1,3]}

Misc Function Decorators

The anyArgs helper is useful for event handlers.

var c = () => "RES";
var anyArgsC = _.anyArgs(c);
anyArgsC(1,2,3); // returns "RES"
anyArgsC(one: 'one', two: 'two'); // equals("RES");

The log helper is useful while debugging.

add(a, b) => a + b;
var addWithLogging = _.log((a,b) => print("arguments are $a and $b"), add);
addWithLogging(1,2) => also prints arguments are 1 and 2

Misc Utils

The symbolize helper returns a map with symbolized keys.

_.symbolize({"one" : 1}); // returns {#one : 1}

The identity returns the given parameter.

_.identity(5); // returns 5

The present returns true if the given object is not null

_.present(5); // returns true
_.present(null); // returns false

fpdart's Issues

Algebraic Types

Any plans on implementing Algebraic Types like Maybe or Either?

Dart SDK version solving failed

when "pub install", fail with message like this:

The current Dart SDK version is 2.1.0-dev.9.4.flutter-f9ebf21297.
Because my_project depends on fp >=0.1.1 which requires SDK version <2.0.0, version solving failed.

so have any plan to support SDK version larger than 2.0.0?

map for Strings

How to iterate over strings? Since Strings are not iterable, functions like _.map don't work. I was thinking maybe this library could define a new class like

class CharList with IterableMixin<String>
{
    List<String> charList;

    Char (String str)
    {
        charList = str.split("");
    }

    //Implement Iterable Interface
}

so you can get consistent iterables from Strings.

Library Documentation

Do you have any plans to update the documentation for the various functions and classes you have implemented?

Test helpers

Hi,

Would it be possible to provide some test helpers? Something like https://pub.dev/packages/dartz_test maybe?

Or is there something out there I missed?

Thanks!