Skip to content
A small functional programming library
Scala Shell
Branch: master
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.


Build Status Download

Coverage Status

A small Functional Programming library. This is not meant to be an alternative to Scalaz or Cats. The reason for having this library is that in your project you don't want to have Scalaz or Cats as its dependency and you only need much smaller set of functional programming features than what Scalaz or Cats offers. So the users of your library can choose Scalaz or Cats to be used with your library.

Getting Started

In build.sbt,

resolvers += "Just Repo" at ""

libraryDependencies += "kevinlee" %% "just-fp" % "1.3.0"

then import

import just.fp._
import just.fp.syntax._


import just.fp._, syntax._

In all the example code using just-fp below, I assume that you've already imported just-fp at the top.


Right-Biased Either

Either in Scala prior to 2.12 is not right-biased meaning that you have to call Either.right all the time if you want to use it with for-comprehension.

e.g.) Before 2.12

for {
  b <- methodReturningEither(a).right
  c <- anotherReturningEither(b).right
} yield c

If you use just-fp, it becomes

for {
  b <- methodReturningEither(a)
  c <- anotherReturningEither(b)
} yield c

Of course, you don't need to do it if you use Scala 2.12 or higher.

Either Constructors

In normal ways, if you want to create Left or Right, you just use the apply methods of their companion objects (i.e. Left() Right()) A problem with this is that what these return is not Either but its data, Left or Right.

You also need to specify not only type parameter for Left but also the one for Right when creating Right.

e.g.) Without type parameters,

// Right[Nothing, Int] = Right(1)

You don't want to have Nothing there. So do it with type parameters,

Right[String, Int](1)
// Right[String, Int] = Right(1)

So it becomes unnecessarily verbose. Right should be inferred as the compiler knows it already yet to specify the left one, you have to put both left and right parameters.

Left, of course, has the same problem.

// Left[String, Nothing] = Left("error")
Left[String, Int]("error")
// Left[String, Int] = Left("error")

Now with just-fp, it's simpler. You can use use left and right constructors as extension methods to the actual data values with only missing type info specified.


1.right[String] // Now you only need to specify
                // the missing type info only
                // that is Left type parameter.
// Either[String, Int] = Right(1)

For Left,

// Either[String, Int] = Left("error")

leftMap and leftFlatMap

So if you Scala 2.12 or higher or just-fp with the older Scala, Either is right-biassed. Then what about the Left case? Can I ever use Left for something useful like transforming the Left value to something else?

For that, just-fp has added leftMap and leftFlatMap to Either. e.g.)

for {
  b <- f1(a).leftMap(AppError.calculatioError)
  c <- f2(b).leftMap(AppError.fromComputeError)
} yield c
// f1 returns Either[String, Int]
// f2 returns Either[ComputeError, Int]
// The result is Either[AppError, Int]


Option Constructors

Similar to Either, creating Option can expose its data instead of type.

e.g.) The following code returns Some[Int] not Option[Int].

// Some[Int] = Some(1)
// None.type = None
// Also None is None not Option[A] 

With just-fp,

// Option[Int] = Some(1)

// Option[String] = None

Type-safe Equal

== in Scala is not type safe so the following code can never be true as their types are different but it has no compile-time error.

1 == "1" // always false, no compile-time error

just-fp has Equal typeclass with typeclass instances for value types (Byte, Short, Int, Char, Long, Float and Double) as well as String, BigInt and BigDecimal.

It also has Equal typeclass instances for WriterT, Writer and EitherT.

With just-fp,

// use triple equals from just-fp
1 === "1"   // compile-time error
1 === 1     // true
"a" === "a" // true
1 !== 1     // false
1 !== 2     // true

If it's a case class the equals of which can be used for equality check, NatualEqual can be used.


case class Foo(n: Int)
Foo(1) === Foo(1)
error: value === is not a member of Foo

This can be solved by NatualEqual.

case class Foo(n: Int)

object Foo {
  implicit val eqaul: Equal[Foo] = Equal.equalA[Foo]

Foo(1) === Foo(1)
// Boolean = true

Foo(1) === Foo(2)
// Boolean = false

Foo(1) !== Foo(1)
// Boolean = false

Foo(1) !== Foo(2)
// Boolean = true


A semi-group is a typeclass which can apply associative binary operation. So if a type is a semi-group, its binary operation append can be applied. It's associative so SemiGroup[A].append(SemiGroup[A].append(a, b), c) is equal to SemiGroup[A].append(a, SemiGroup[A].append(b, c))


def foo[A](x: Int, y: Int, f: Int => A)(implicit S: SemiGroup[A]): A =
  S.append(f(x), f(y))

// or with context bound
def foo[A: SemiGroup](x: Int, y: Int, f: Int => A): A =
  implicitly[SemiGroup[A]].append(f(x), f(y))

If there is a typeclass instance of SemiGroup for a type A, mappend method or a convenient |+| infix operator can be used like this.

e.g.) There is already a SemiGroup typeclass instance for Int in just-fp so you can do

// Int = 3

1 |+| 2
// Int = 3

Typeclass instances for the following typeclasses are available in just-fp.

  • SemiGroup[List[A]]
    List(1, 2, 3) |+| List(4, 5, 6)
    // List[Int] = List(1, 2, 3, 4, 5, 6)
    List(1, 2, 3).mappend(List(4, 5, 6))
    // List[Int] = List(1, 2, 3, 4, 5, 6)
  • SemiGroup[Vector[A]]
    Vector(1, 2, 3) |+| Vector(4, 5, 6)
    // Vector[Int] = Vector(1, 2, 3, 4, 5, 6)
    Vector(1, 2, 3).mappend(Vector(4, 5, 6))
    // Vector[Int] = Vector(1, 2, 3, 4, 5, 6)
  • SemiGroup[String]
    "abc" |+| "def"
    // String = "abcdef"
    // String = "abcdef"
  • SemiGroup[Byte]
    1.toByte |+| 2.toByte
    // Byte = 3
    // Byte = 3
  • SemiGroup[Short]
    1.toShort |+| 2.toShort
    // Short = 3
    // Short = 3
  • SemiGroup[Char]
    'A' |+| '1'
    // Char = 'r'
    // Char = 'r'
  • SemiGroup[Int]
    1 |+| 2
    // Int = 3
    // Int = 3
  • SemiGroup[Long]
    1L |+| 2L
    // Long = 3L
    // Long = 3L
  • SemiGroup[BigInt]
    BigInt(1) |+| BigInt(2)
    // BigInt = 3
    // BigInt = 3
  • SemiGroup[BigDecimal]
    BigDecimal(1) |+| BigDecimal(2)
    // BigDecimal = 3
    // BigDecimal = 3
  • SemiGroup[Option[A]] if there is a typeclass instance of SemiGroup[A].
    1.some |+| 2.some
    // Option[Int] = Some(3)
    // Option[Int] = Some(3)
    NOTE: There might be an associativity issue with BigDecimal if it's created with other MathContext than MathContext.UNLIMITED and the number is big enough in Scala 2.13. More about the issue please read this blog.


// To be updated ...


// To be updated ...


// To be updated ...


// To be updated ...

WriterT / Writer

// To be updated ...


// To be updated ...

You can’t perform that action at this time.