package scodec

Combinator library for working with binary data.

The primary abstraction of this library is Codec, which provides the ability to encode/decode values to/from binary.

There are more general abstractions though, such as Encoder and Decoder. There's also GenCodec which extends both Encoder and Decoder but allows the types to vary. Given these more general abstractions, a Codec[A] can be represented as a GenCodec[A, A].

The more general abstractions are important because they allow operations on codecs that would not otherwise be possible. For example, given a Codec[A], mapping a function A => B over the codec yields a GenCodec[A, B]. Without the more general abstractions, map is impossible to define (e.g., how would be implemented?). Given a GenCodec[A, B], the encoding functionality can be ignored by treating it as a Decoder[B], or the encoding type can be changed via contramap. If after further transformations, the two types to GenCodec are equal, we can reconstitute a Codec from the GenCodec by calling fuse.

See the codecs package object for pre-defined codecs for many common data types and combinators for building larger codecs out of smaller ones.

For the categorically minded, note the following:

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  1. sealed abstract class Attempt[+A] extends Product with Serializable

    Right biased Either[Err, A].

    Right biased Either[Err, A].

    An Attempt is either an Attempt.Successful or an Attempt.Failure. Attempts can be created by calling Attempt.successful or Attempt.failure, as well as converting from an Option via fromOption.

  2. trait Codec[A] extends GenCodec[A, A]

    Supports encoding a value of type A to a BitVector and decoding a BitVector to a value of A.

    Supports encoding a value of type A to a BitVector and decoding a BitVector to a value of A.

    Not every value of A can be encoded to a bit vector and similarly, not every bit vector can be decoded to a value of type A. Hence, both encode and decode return either an error or the result. Furthermore, decode returns the remaining bits in the bit vector that it did not use in decoding.

    There are various ways to create instances of Codec. The trait can be implemented directly or one of the constructor methods in the companion can be used (e.g., apply). Most of the methods on Codec create return a new codec that has been transformed in some way. For example, the xmap method converts a Codec[A] to a Codec[B] given two functions, A => B and B => A.

    One of the simplest transformation methods is def withContext(context: String): Codec[A], which pushes the specified context string in to any errors (i.e., Errs) returned from encode or decode.

    See the methods on this trait for additional transformation types.

    See the codecs package object for pre-defined codecs for many common data types and combinators for building larger codecs out of smaller ones.

    Tuple Codecs

    The ~ operator supports combining a Codec[A] and a Codec[B] in to a Codec[(A, B)].

    For example:

    val codec: Codec[Int ~ Int ~ Int] = uint8 ~ uint8 ~ uint8

    Codecs generated with ~ result in left nested tuples. These left nested tuples can be pulled back apart by pattern matching with ~. For example:

    Codec.decode(uint8 ~ uint8 ~ uint8, bytes) map { case a ~ b ~ c => a + b + c }

    Alternatively, a function of N arguments can be lifted to a function of left-nested tuples. For example:

    val add3 = (_: Int) + (_: Int) + (_: Int)
    Codec.decode(uint8 ~ uint8 ~ uint8, bytes) map add3

    Similarly, a left nested tuple can be created with the ~ operator. This is useful when creating the tuple structure to pass to encode. For example:

    (uint8 ~ uint8 ~ uint8).encode(1 ~ 2 ~ 3)

    Tuple based codecs are of limited use compared to HList based codecs, which is discussed later.

    Note: this design is heavily based on Scala's parser combinator library and the syntax it provides.


    Sometimes when combining codecs, a latter codec depends on a formerly decoded value. The flatZip method is important in these types of situations -- it represents a dependency between the left hand side and right hand side. Its signature is def flatZip[B](f: A => Codec[B]): Codec[(A, B)]. This is similar to flatMap except the return type is Codec[(A, B)] instead of Decoder[B].

    Consider a binary format of an 8-bit unsigned integer indicating the number of bytes following it. To implement this with flatZip, we could write:

    val x: Codec[(Int, ByteVector)] = uint8 flatZip { numBytes => bytes(numBytes) }
    val y: Codec[ByteVector] = x.xmap[ByteVector]({ case (_, bv) => bv }, bv => (bv.size, bv))

    In this example, x is a Codec[(Int, ByteVector)] but we do not need the size directly in the model because it is redundant with the size stored in the ByteVector. Hence, we remove the Int by xmap-ping over x. The notion of removing redundant data from models comes up frequently. Note: there is a combinator that expresses this pattern more succinctly -- variableSizeBytes(uint8, bytes).

    HList Codecs

    HLists are similar to tuples in that they represent the product of an arbitrary number of types. That is, the size of an HList is known at compile time and the type of each element is also known at compile time. For more information on HLists in general, see Shapeless.

    Codec makes heavy use of HLists. The primary operation is extending a Codec[L] for some L <: HList to a Codec[A :: L]. For example:

    val uint8: Codec[Int] = ...
    val string: Codec[String] = ...
    val codec: Codec[Int :: Int :: String] = uint8 :: uint8 :: string

    The :: method is sort of like cons-ing on to the HList but it is doing so *inside* the Codec type. The resulting codec encodes values by passing each component of the HList to the corresponding codec and concatenating all of the results.

    There are various methods on this trait that only work on Codec[L] for some L <: HList. Besides the aforementioned :: method, there are others like :::, flatPrepend, flatConcat, etc. One particularly useful method is dropUnits, which removes any Unit values from the HList.

    Given a Codec[X0 :: X1 :: ... Xn :: HNil] and a case class with types X0 to Xn in the same order, the HList codec can be turned in to a case class codec via the as method. For example:

    case class Point(x: Int, y: Int, z: Int)
    val threeInts: Codec[Int :: Int :: Int :: HNil] = uint8 :: uint8 :: uint8
    val point: Codec[Point] =[Point]

    The HList analog to flatZip is flatPrepend. It has the signature:

    def flatPrepend[L <: HList](f: A => Codec[L]): Codec[A :: L]

    It forms a codec of A consed on to L when called on a Codec[A] and passed a function A => Codec[L]. Note that the specified function must return an HList based codec. Implementing our example from earlier using flatPrepend:

    val x: Codec[Int :: ByteVector :: HNil] = uint8 flatPrepend { numBytes => bytes(numBytes).hlist }

    In this example, bytes(numBytes) returns a Codec[ByteVector] so we called .hlist on it to lift it in to a Codec[ByteVector :: HNil].

    There are similar methods for flat appending and flat concating.

    Coproduct Codecs

    Given some ordered list of types, potentially with duplicates, a value of the HList of those types has a value for *every* type in the list. In other words, an HList represents having an X0 AND X1 AND ... AND XN. A Coproduct for the same list of types represents having a value for *one* of those types. In other words, a Coproduct represents having an X0 OR X1 OR ... OR XN. This is somewhat imprecise because a coproduct can tell us exactly which Xi we have, even in the presence of duplicate types.

    A coproduct can also be thought of as an Either that has an unlimited number of choices instead of just 2 choices.

    Shapeless represents coproducts in a similar way as HLists. A coproduct type is built using the :+: operator with a sentinal value of CNil. For example, an Int or Long or String is represented as the coproduct type:

    Int :+: Long :+: String :+: CNil

    For more information on coproducts in general, see Shapeless.

    Like HList based codecs, scodec supports Coproduct based codecs by coopting syntax from Shapeless. Specifically, the :+: operator is used:

    val builder = uint8 :+: int64 :+: utf8

    Unlike HList based codecs, the result of :+: is not a codec but rather a codecs.CoproductCodecBuilder. Having a list of types and a codec for each is not sufficient to build a coproduct codec. We also need to describe how each entry in the coproduct is differentiated from the other entries. There are a number of ways to do this and each way changes the binary format significantly. See the docs on CoproductCodecBuilder for details.

    Derived Codecs

    Codecs for case classes and sealed class hierarchies can often be automatically derived.

    Consider this example:

    import scodec.codecs.implicits._
    case class Point(x: Int, y: Int, z: Int)
    Codec[Point].encode(Point(1, 2, 3))

    In this example, no explicit codec was defined for Point yet Codec[Point] successfully created one. It did this by "reflecting" over the structure of Point and looking up a codec for each component type (note: no runtime reflection is performed - rather, this is implemented using macro-based compile time reflection). In this case, there are three components, each of type Int, so the compiler first looked for an implicit Codec[Int]. It then combined each Codec[Int] using an HList based codec and finally converted the HList codec to a Codec[Point]. It found the implicit Codec[Int] instances due to the import of scodec.codecs.implicits._. Furthermore, if there was an error encoding or decoding a field, the field name (i.e., x, y, or z) is included as context on the Err returned.

    This works similarly for sealed class hierarchies -- each subtype is internally represented as a member of a coproduct. There must be the following implicits in scope however:

    • Discriminated[A, D] for some discriminator type D, which provides the Codec[D] to use for encoding/decoding the discriminator
    • Discriminator[A, X, D] for each subtype X of A, which provides the discriminator value for type X
    • Codec[X] for each subtype X of A

    Full examples are available in the test directory of this project.

    Implicit Codecs

    If authoring combinators that require implicit codec arguments, use shapeless.Lazy[Codec[A]] instead of Codec[A]. This prevents the occurrence of diverging implicit expansion errors.


    Note: DecodingContext allows multiple decoders/codecs to be sequenced one after the other, with the remainder from each decode operation fed in to the input of the next.

  3. final case class DecodeResult[+A](value: A, remainder: BitVector) extends Product with Serializable

    Result of a decoding operation, which consists of the decoded value and the remaining bits that were not consumed by decoding.

  4. trait Decoder[+A] extends AnyRef

    Supports decoding a value of type A from a BitVector.

  5. trait DecoderFunctions extends AnyRef

    Provides functions for working with decoders.

  6. sealed abstract class DecodingContext[A] extends AnyRef

    Provides the ability to sequence decoding operations such that the remainder of an operation is fed in to the input of the next operation.

    Provides the ability to sequence decoding operations such that the remainder of an operation is fed in to the input of the next operation. This is useful when using codecs in for comprehensions for decoding purposes.

    Note: this is a domain specific fail fast state monad.

  7. trait Encoder[-A] extends AnyRef

    Supports encoding a value of type A to a BitVector.

  8. trait EncoderFunctions extends AnyRef

    Provides functions for working with encoders.

  9. implicit final class EnrichedCoproductDecoder[C <: Coproduct] extends AnyVal

    Provides methods specific to decoders of Shapeless coproducts.

  10. implicit final class EnrichedCoproductEncoder[C <: Coproduct] extends AnyVal

    Provides methods specific to encoders of Shapeless coproducts.

  11. implicit final class EnrichedHList[L <: HList] extends AnyVal

    Provides additional methods on HLists.

  12. trait Err extends AnyRef

    Describes an error.

    Describes an error.

    An error has a message and a list of context identifiers that provide insight into where an error occurs in a large structure.

    This type is not sealed so that codecs can return domain specific subtypes and dispatch on those subtypes.

  13. trait GenCodec[-A, +B] extends Encoder[A] with Decoder[B]

    Generalized codec that allows the type to encode to vary from the type to decode.

  14. implicit final class HListCodecEnrichedWithHListSupport[L <: HList] extends AnyVal

    Provides common operations on a Codec[HList].

  15. final case class SizeBound extends Product with Serializable

    Bounds the size, in bits, of the binary encoding of a codec -- i.e., it provides a lower bound and an upper bound on the size of bit vectors returned as a result of encoding.

  16. abstract class Transform[F[_]] extends AnyRef

    Typeclass that describes type constructors that support the exmap operation.

  17. implicit class TransformSyntax[F[_], A] extends AnyRef

    Provides method syntax for working with a type constructor that has a Transform typeclass instance.

  18. abstract class Transformer[A, B] extends AnyRef

    Witness operation that supports transforming an F[A] to an F[B] for all F which have a Transform instance available.

    Witness operation that supports transforming an F[A] to an F[B] for all F which have a Transform instance available.

    @implicitNotFound( ... )
  19. sealed abstract class TransformerLowPriority extends AnyRef

    Low priority Transformer builders.

  20. implicit final class Tuple2CodecSupport[A] extends AnyVal

  21. implicit final class ValueCodecEnrichedWithGenericSupport[A] extends AnyVal

    Provides syntax related to generic programming for codecs of any type.

  22. implicit final class ValueCodecEnrichedWithHListSupport[A] extends AnyVal

    Provides HList related syntax for codecs of any type.

Value Members

  1. object Attempt extends Serializable

    Companion for Attempt.

  2. object BuildInfo extends Product with Serializable

    This object was generated by sbt-buildinfo.

  3. object Codec extends EncoderFunctions with DecoderFunctions

    Companion for Codec.

  4. object Decoder extends DecoderFunctions

    Companion for Decoder.

  5. object DecodingContext

    Provides constructors for DecodingContext.

  6. object Encoder extends EncoderFunctions

    Companion for Encoder.

  7. object Err

    Companion for Err.

  8. object GenCodec extends EncoderFunctions with DecoderFunctions

    Companion for GenCodec.

  9. object SizeBound extends Serializable

    Companion for SizeBound.

  10. object Transform

    Companion for Transform.

  11. object Transformer extends TransformerLowPriority

    Companion for Transformer.

  12. package codecs

    Provides codecs for common types and combinators for building larger codecs.

    Provides codecs for common types and combinators for building larger codecs.

    Bits and Bytes Codecs

    The simplest of the provided codecs are those that encode/decode BitVectors and ByteVectors directly. These are provided by bits and bytes methods. These codecs encode all of the bits/bytes directly in to the result and decode *all* of the remaining bits/bytes in to the result value. That is, the result of decode always returns a empty bit vector for the remaining bits.

    Similarly, fixed size alternatives are provided by the bits(size) and bytes(size) methods, which encode a fixed number of bits/bytes (or error if not provided the correct size) and decoded a fixed number of bits/bytes (or error if that many bits/bytes are not available).

    There are more specialized codecs for working with bits, including ignore and constant.

    Numeric Codecs

    There are built-in codecs for Int, Long, Float, and Double.

    There are a number of predefined integral codecs named using the form:


    where u stands for unsigned, size is replaced by one of 8, 16, 24, 32, 64, and L stands for little-endian. For each codec of that form, the type is Codec[Int] or Codec[Long] depending on the specified size. For example, int32 supports 32-bit big-endian 2s complement signed integers, and uint16L supports 16-bit little-endian unsigned integers. Note: uint64[L] are not provided because a 64-bit unsigned integer does not fit in to a Long.

    Additionally, methods of the form [u]int[L](size: Int) and [u]long[L](size: Int) exist to build arbitrarily sized codecs, within the limitations of Int and Long.

    IEEE 754 floating point values are supported by the float, floatL, double, and doubleL codecs.

    Miscellaneous Value Codecs

    In addition to the numeric codecs, there are built-in codecs for Boolean, String, and UUID.

    Boolean values are supported by the bool codecs.


    There are a number of methods provided that create codecs out of other codecs. These include simple combinators such as fixedSizeBits and variableSizeBits and advanced combinators such as discriminated, which provides its own DSL for building a large codec out of many small codecs. For a list of all combinators, see the Combinators section below.

    Cryptography Codecs

    There are codecs that support working with encrypted data (encrypted), digital signatures and checksums (fixedSizeSignature and variableSizeSignature). Additionally, support for is provided by certificate and x509Certificate.

Inherited from AnyRef

Inherited from Any