scodec

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.

Companion for Attempt.

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

There are various methods on Codec that only work on Codec[A] for some A <: Tuple. Besides the aforementioned :: method, they include methods like ++, flatPrepend, flatConcat, etc. One particularly useful method is dropUnits, which removes any Unit values from the tuple.

Given a Codec[(X0, X1, ..., Xn)] and a case class with types X0 to Xn in the same order, the 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)] = uint8 :: uint8 :: uint8
  val point: Codec[Point] = threeInts.as[Point]

flatZip

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).

flatPrepend

When the function passed to flatZip returns a Codec[B] where B <: Tuple, you end up creating right nested tuples instead of a extending the arity of a single tuple. To do the latter, there's flatPrepend. It has the signature:

  def flatPrepend[B <: Tuple](f: A => Codec[B]): Codec[A *: B]

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

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

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

There are similar methods for flat appending and flat concating.

Derived Codecs

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

Consider this example:

  case class Point(x: Int, y: Int, z: Int) derives Codec
  Codec[Point].encode(Point(1, 2, 3))

In this example, no explicit codec was defined for Point and instead, one was derived as a result of the derives Codec clause. Derivation of a codec for a case class requires each element of the case class to have a given codec of the corresponding type. In this case, each element was an Int and there is a Codec[Int] given in the companion of Codec.

Derived codecs include the name of each element in any errors produced when encoding/decoding the element.

This works similarly for ADTs / sealed class hierarchies. The binary form is represented as a single unsigned 8-bit integer representing the ordinal of the sum, followed by the derived form of the product.

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

Companion for Codec.

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

Supports decoding a value of type A from a BitVector.

Companion for Decoder.

Provides functions for working with decoders.

Supports encoding a value of type A to a BitVector.

Companion for Encoder.

Provides functions for working with encoders.

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.

Companion for Err.

Companion for Iso.

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.

Companion for SizeBound.

Typeclass that describes type constructors that support the exmap operation.

The tuple which is the result of removing all 'Unit' types from the tuple 'A'.