scodec
package scodec
Type members
Classlikes
Supports encoding a value of type
A
to a BitVector
and decoding a BitVector
to a value of A
.Not every value of
of type
remaining bits in the bit vector that it did not use in decoding.
A
can be encoded to a bit vector and similarly, not every bit vector can be decoded to a valueof type
A
. Hence, both encode and decode return either an error or the result. Furthermore, decode returns theremaining bits in the bit vector that it did not use in decoding.
There are various ways to create instances of
constructor methods in the companion can be used (e.g.,
create return a new codec that has been transformed in some way. For example, the xmap method
converts a
Codec
. The trait can be implemented directly or one of theconstructor 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
pushes the specified context string in to any errors (i.e.,
def withContext(context: String): Codec[A]
, whichpushes the specified context string in to any errors (i.e.,
Err
s) 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.
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
}}}
}}}
{{{
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 isdropUnits
, which removes any Unit
values from the tuple.Given a
the codec can be turned in to a case class codec via the
{{{
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]
}}}
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
the left hand side and right hand side. Its signature is
This is similar to
The
flatZip
method is important in these types of situations -- it represents a dependency betweenthe 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
{{{
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,
because it is redundant with the size stored in the
Note: there is a combinator that expresses this pattern more succinctly --
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 modelbecause it is redundant with the size stored in the
ByteVector
. Hence, we remove the Int
byxmap
-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
right nested tuples instead of a extending the arity of a single tuple. To do the latter, there's
{{{
def flatPrepend[B <: Tuple] (f: A => Codec[B] ): Codec[A *: B]
}}}
It forms a codec of
Note that the specified function must return a tuple codec. Implementing our example from earlier
using
{{{
val x: Codec[(Int, ByteVector)] = uint8.flatPrepend { numBytes => bytes(numBytes).tuple }
}}}
In this example,
in to a
flatZip
returns a Codec[B]
where B <: Tuple
, you end up creatingright 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 itin 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
of the
have an implicitly available codec of the corresponding type. In this case, each element was an
an implicit
{{{
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, an implicit one was derived as a resultof the
derives Codec
clause. Derivation of a codec for a case class requires each element of the case class tohave an implicitly available codec of the corresponding type. In this case, each element was an
Int
and there isan implicit
Codec[Int]
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.
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
- object
trait Err
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.
subtypes and dispatch on those subtypes.
- Companion
- object
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.
of bit vectors returned as a result of encoding.
- Value Params
- lowerBound
-
Minimum number of bits
- upperBound
-
Maximum number of bits
- Companion
- object