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