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fthomas/refined

Refinement types for Scala
Source code at GitHub https://github.com/fthomas/refined
fthomas/refined.json
{
  "createdAt": "2015-05-10T16:23:16Z",
  "defaultBranch": "master",
  "description": "Refinement types for Scala",
  "fullName": "fthomas/refined",
  "homepage": "",
  "language": "Scala",
  "name": "refined",
  "pushedAt": "2025-11-26T17:27:23Z",
  "stargazersCount": 1725,
  "topics": [
    "refinement-types",
    "scala",
    "typelevel"
  ],
  "updatedAt": "2025-11-26T17:27:27Z",
  "url": "https://github.com/fthomas/refined"
}

refined: simple refinement types for Scala

Section titled “refined: simple refinement types for Scala”

GitHub Workflow Status codecov.io Gitter refined Scala version support Scaladoc

refined is a Scala library for refining types with type-level predicates which constrain the set of values described by the refined type. It started as a port of the [refined][refined.hs] Haskell library by Nikita Volkov (which also provides an excellent motivation why this kind of library is useful). The idea of expressing constraints at the type-level as Scala library was first explored by Flavio W. Brasil in [bond][bond].

A quick example:

import eu.timepit.refined._
import eu.timepit.refined.api.Refined
import eu.timepit.refined.auto._
import eu.timepit.refined.numeric._


// This refines Int with the Positive predicate and checks via an
// implicit macro that the assigned value satisfies it:
scala> val i1: Int Refined Positive = 5
i1: Int Refined Positive = 5


// If the value does not satisfy the predicate, we get a meaningful
// compile error:
scala> val i2: Int Refined Positive = -5
<console>:22: error: Predicate failed: (-5 > 0).
       val i2: Int Refined Positive = -5
                                       ^


// There is also the explicit refineMV macro that can infer the base
// type from its parameter:
scala> refineMV[Positive]!(5)
res0: Int Refined Positive = 5


// Macros can only validate literals because their values are known at
// compile-time. To validate arbitrary (runtime) values we can use the
// refineV function:


scala> val x = 42 // suppose the value of x is not known at compile-time


scala> refineV[Positive]!(x)
res1: Either[String, Int Refined Positive] = Right(42)


scala> refineV[Positive]!(-x)
res2: Either[String, Int Refined Positive] = Left(Predicate failed: (-42 > 0).)

refined also contains inference rules for converting between different refined types. For example, Int Refined Greater[10] can be safely converted to Int Refined Positive because all integers greater than ten are also positive. The type conversion of refined types is a compile-time operation that is provided by the library:

scala> val a: Int Refined Greater[5] = 10
a: Int Refined Greater[Int(5)] = 10


// Since every value greater than 5 is also greater than 4, `a` can be
// ascribed the type Int Refined Greater[4] !:
scala> val b: Int Refined Greater[4] = a
b: Int Refined Greater[Int(4)] = 10


// An unsound ascription leads to a compile error:
scala> val c: Int Refined Greater[6] = a
                                       ^
       error: type mismatch (invalid inference):
               eu.timepit.refined.numeric.Greater[5] does not imply
               eu.timepit.refined.numeric.Greater[6]

This mechanism allows to pass values of more specific types (e.g. Int Refined Greater[10]) to functions that take a more general type (e.g. Int Refined Positive) without manual intervention.

prior Scala 2.13 without literal types

Section titled “prior Scala 2.13 without literal types”

Since there are no literal types prior to Scala 2.13 the literals must be created with shapeless:

scala> val a: Int Refined Greater[W.`5`.T] = 10
a: Int Refined Greater[Int(5)] = 10
scala> val b: Int Refined Greater[W.`4`.T] = a
b: Int Refined Greater[Int(4)] = 10

Note that W is a shortcut for [shapeless.Witness][singleton-types] which provides syntax for [literal-based singleton types][sip-23].

Table of contents

Section titled “Table of contents”
  1. More examples
  2. Using refined
  3. Community
  4. Documentation
  5. Provided predicates
  6. Contributors and participation
  7. Related projects
  8. License

More examples

Section titled “More examples”
import eu.timepit.refined._
import eu.timepit.refined.auto._
import eu.timepit.refined.numeric._
import eu.timepit.refined.api.{RefType, Refined}
import eu.timepit.refined.boolean._
import eu.timepit.refined.char._
import eu.timepit.refined.collection._
import eu.timepit.refined.generic._
import eu.timepit.refined.string._
import shapeless.{ ::, HNil }


scala> refineMV[NonEmpty]!("Hello")
res2: String Refined NonEmpty = Hello


scala> refineMV[NonEmpty]!("")
<console>:39: error: Predicate isEmpty() did not fail.
            refineMV[NonEmpty]!("")
                              ^


scala> type ZeroToOne = Not[Less[0.0]] And Not[Greater[1.0]]
defined type alias ZeroToOne


scala> refineMV[ZeroToOne]!(1.8)
<console>:40: error: Right predicate of (!(1.8 < 0.0) && !(1.8 > 1.0)) failed: Predicate (1.8 > 1.0) did not fail.
       refineMV[ZeroToOne]!(1.8)
                          ^


scala> refineMV[AnyOf[Digit :: Letter :: Whitespace :: HNil]]!('F')
res3: Char Refined AnyOf[Digit :: Letter :: Whitespace :: HNil] = F


scala> refineMV[MatchesRegex["[0-9]+"]]!("123.")
<console>:39: error: Predicate failed: "123.".matches("[0-9]+").
              refineMV[MatchesRegex[W.`"[0-9]+"`.T]]!("123.")
                                                    ^


scala> val d1: Char Refined Equal['3'] = '3'
d1: Char Refined Equal[Char('3')] = 3


scala> val d2: Char Refined Digit = d1
d2: Char Refined Digit = 3


scala> val d3: Char Refined Letter = d1
<console>:39: error: type mismatch (invalid inference):
 Equal[Char('3')] does not imply
 Letter
       val d3: Char Refined Letter = d1
                                     ^


scala> val r1: String Refined Regex = "(a|b)"
r1: String Refined Regex = (a|b)


scala> val r2: String Refined Regex = "(a|b"
<console>:38: error: Regex predicate failed: Unclosed group near index 4
(a|b
    ^
       val r2: String Refined Regex = "(a|b"
                                      ^


scala> val u1: String Refined Url = "htp://example.com"
<console>:38: error: Url predicate failed: unknown protocol: htp
       val u1: String Refined Url = "htp://example.com"
                                    ^


// Here we define a refined type "Int with the predicate (7 <= value < 77)".
scala> type Age = Int Refined Interval.ClosedOpen[7, 77]


scala> val userInput = 55


// We can refine values with this refined type by either using `refineV`
// with an explicit return type
scala> val ageEither1: Either[String, Age] = refineV(userInput)
ageEither1: Either[String,Age] = Right(55)


// or by using `RefType.applyRef` with the refined type as type parameter.
scala> val ageEither2 = RefType.applyRef[Age]!(userInput)
ageEither2: Either[String,Age] = Right(55)

Using refined

Section titled “Using refined”

The latest version of the library is 0.11.3, which is available for Scala and [Scala.js][scala.js] version 2.12 and 2.13.

If you’re using sbt, add the following to your build:

libraryDependencies ++= Seq(
  "eu.timepit" %% "refined"                 % "0.11.3",
  "eu.timepit" %% "refined-cats"            % "0.11.3", // optional
  "eu.timepit" %% "refined-eval"            % "0.11.3", // optional, JVM-only
  "eu.timepit" %% "refined-jsonpath"        % "0.11.3", // optional, JVM-only
  "eu.timepit" %% "refined-pureconfig"      % "0.11.3", // optional, JVM-only
  "eu.timepit" %% "refined-scalacheck"      % "0.11.3", // optional
  "eu.timepit" %% "refined-scalaz"          % "0.11.3", // optional
  "eu.timepit" %% "refined-scodec"          % "0.11.3", // optional
  "eu.timepit" %% "refined-scopt"           % "0.11.3", // optional
  "eu.timepit" %% "refined-shapeless"       % "0.11.3"  // optional
)

For Scala.js just replace %% with %%% above.

Instructions for Maven and other build tools are available at [search.maven.org][search.maven].

Release notes for the latest version are here.

Community

Section titled “Community”

Internal modules

Section titled “Internal modules”

The project provides these optional extensions and library integrations:

  • refined-cats provides Cats type class instances for refined types
  • refined-eval provides the Eval[S] predicate that checks if a value applied to the predicate S yields true
  • refined-jsonpath provides the JSONPath predicate that checks if a String is a valid JSONPath
  • refined-pureconfig allows to read configuration with refined types using PureConfig
  • refined-scalacheck allows to generate arbitrary values of refined types with ScalaCheck. Use refined-scalacheck_1.13 instead if your other dependencies use scalacheck version 1.13
  • refined-scalaz provides Scalaz type class instances for refined types and support for scalaz.@@
  • refined-scodec allows binary decoding and encoding of refined types with scodec and allows refining scodec.bits.ByteVector
  • refined-scopt allows to read command line options with refined types using scopt
  • refined-shapeless

External modules

Section titled “External modules”

Below is an incomplete list of third-party extensions and library integrations for refined. If your library is missing, please open a pull request to list it here:

Projects using refined

Section titled “Projects using refined”

If your open source project is using refined, please consider opening a pull request to list it here:

Adopters

Section titled “Adopters”

Are you using refined in your organization or company? Please consider opening a pull request to list it here:

Documentation

Section titled “Documentation”

API documentation of the latest release is available at: https://static.javadoc.io/eu.timepit/refined_2.12/0.11.3/eu/timepit/refined/index.html

There are further (type-checked) examples in the [docs][docs] directory including ones for defining [custom predicates][custom-pred] and working with [type aliases][type-aliases]. It also contains a [description][design] of refined’s design and internals.

Talks and other external resources are listed on the [Resources][resources] page in the wiki.

[custom-pred] !: https://github.com/fthomas/refined/blob/master/modules/docs/custom_predicates.md [design] !: https://github.com/fthomas/refined/blob/master/modules/docs/design.md [docs] !: https://github.com/fthomas/refined/tree/master/modules/docs [resources] !: https://github.com/fthomas/refined/wiki/Resources [type-aliases] !: https://github.com/fthomas/refined/blob/master/modules/docs/type_aliases.md

Provided predicates

Section titled “Provided predicates”

The library comes with these predefined predicates:

boolean

  • True: constant predicate that is always true
  • False: constant predicate that is always false
  • Not[P]: negation of the predicate P
  • And[A, B]: conjunction of the predicates A and B
  • Or[A, B]: disjunction of the predicates A and B
  • Xor[A, B]: exclusive disjunction of the predicates A and B
  • Nand[A, B]: negated conjunction of the predicates A and B
  • Nor[A, B]: negated disjunction of the predicates A and B
  • AllOf[PS]: conjunction of all predicates in PS
  • AnyOf[PS]: disjunction of all predicates in PS
  • OneOf[PS]: exclusive disjunction of all predicates in PS

char

  • Digit: checks if a Char is a digit
  • Letter: checks if a Char is a letter
  • LetterOrDigit: checks if a Char is a letter or digit
  • LowerCase: checks if a Char is a lower case character
  • UpperCase: checks if a Char is an upper case character
  • Whitespace: checks if a Char is white space

collection

  • Contains[U]: checks if an Iterable contains a value equal to U
  • Count[PA, PC]: counts the number of elements in an Iterable which satisfy the predicate PA and passes the result to the predicate PC
  • Empty: checks if an Iterable is empty
  • NonEmpty: checks if an Iterable is not empty
  • Forall[P]: checks if the predicate P holds for all elements of an Iterable
  • Exists[P]: checks if the predicate P holds for some elements of an Iterable
  • Head[P]: checks if the predicate P holds for the first element of an Iterable
  • Index[N, P]: checks if the predicate P holds for the element at index N of a sequence
  • Init[P]: checks if the predicate P holds for all but the last element of an Iterable
  • Last[P]: checks if the predicate P holds for the last element of an Iterable
  • Tail[P]: checks if the predicate P holds for all but the first element of an Iterable
  • Size[P]: checks if the size of an Iterable satisfies the predicate P
  • MinSize[N]: checks if the size of an Iterable is greater than or equal to N
  • MaxSize[N]: checks if the size of an Iterable is less than or equal to N

generic

  • Equal[U]: checks if a value is equal to U

numeric

  • Less[N]: checks if a numeric value is less than N
  • LessEqual[N]: checks if a numeric value is less than or equal to N
  • Greater[N]: checks if a numeric value is greater than N
  • GreaterEqual[N]: checks if a numeric value is greater than or equal to N
  • Positive: checks if a numeric value is greater than zero
  • NonPositive: checks if a numeric value is zero or negative
  • Negative: checks if a numeric value is less than zero
  • NonNegative: checks if a numeric value is zero or positive
  • Interval.Open[L, H]: checks if a numeric value is in the interval (L, H)
  • Interval.OpenClosed[L, H]: checks if a numeric value is in the interval (L, H]
  • Interval.ClosedOpen[L, H]: checks if a numeric value is in the interval [L, H)
  • Interval.Closed[L, H]: checks if a numeric value is in the interval [L, H]
  • Modulo[N, O]: checks if an integral value modulo N is O
  • Divisible[N]: checks if an integral value is evenly divisible by N
  • NonDivisible[N]: checks if an integral value is not evenly divisible by N
  • Even: checks if an integral value is evenly divisible by 2
  • Odd: checks if an integral value is not evenly divisible by 2
  • NonNaN: checks if a floating-point number is not NaN

string

  • EndsWith[S]: checks if a String ends with the suffix S
  • IPv4: checks if a String is a valid IPv4
  • IPv6: checks if a String is a valid IPv6
  • MatchesRegex[S]: checks if a String matches the regular expression S
  • Regex: checks if a String is a valid regular expression
  • StartsWith[S]: checks if a String starts with the prefix S
  • Uri: checks if a String is a valid URI
  • Url: checks if a String is a valid URL
  • Uuid: checks if a String is a valid UUID
  • ValidByte: checks if a String is a parsable Byte
  • ValidShort: checks if a String is a parsable Short
  • ValidInt: checks if a String is a parsable Int
  • ValidLong: checks if a String is a parsable Long
  • ValidFloat: checks if a String is a parsable Float
  • ValidDouble: checks if a String is a parsable Double
  • ValidBigInt: checks if a String is a parsable BigInt
  • ValidBigDecimal: checks if a String is a parsable BigDecimal
  • Xml: checks if a String is well-formed XML
  • XPath: checks if a String is a valid XPath expression
  • Trimmed: checks if a String has no leading or trailing whitespace
  • HexStringSpec: checks if a String represents a hexadecimal number

Contributors and participation

Section titled “Contributors and participation”

The following people have helped making refined great:

refined is a [Typelevel][typelevel] project. This means we embrace pure, typeful, functional programming, and provide a safe and friendly environment for teaching, learning, and contributing as described in the Scala Code of Conduct.

Section titled “Related projects”

License

Section titled “License”

refined is licensed under the MIT license, available at http://opensource.org/licenses/MIT and also in the LICENSE file.

[bond] !: https://github.com/fwbrasil/bond [refined.hs] !: http://nikita-volkov.github.io/refined [scala.js] !: http://www.scala-js.org [search.maven] !: http://search.maven.org/#search|ga|1|eu.timepit.refined [singleton-types] !: https://github.com/milessabin/shapeless/wiki/Feature-overview:-shapeless-2.0.0#singleton-typed-literals [sip-23] !: http://docs.scala-lang.org/sips/42.type.html [typelevel] !: http://typelevel.org