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Fressian is an extensible binary data notation. Fressian is used by Datomic and other applications as a data transfer format. This spec describes Fressian in isolation from those and other specific use cases, to help facilitate implementation of readers and writers in other languages, and for other uses.

Fressian supports a rich set of built-in elements, and the definition of extension elements in terms of the others. Users of data formats without such facilities must rely on either convention or context to convey elements not included in the base set. This greatly complicates application logic, betraying the apparent simplicity of the format. Fressian is simple, yet powerful enough to meet the demands of applications without convention or complex context-sensitive logic.

Fressian is a counterpart to edn, and shares many of the same design objects. The key additional objective that separates Fressian from edn is efficiency. To that end Fressian:

  • is a binary format
  • directly supports platform primitive types
  • directly supports platform arrays
  • enables inline caching

General considerations

Fressian is a byte code specification. ...

Packed representations

TODO describe packing


TODO describe, very similar to struct definitions below,
     as structs are implicitly autocached


Fressian writers can define new struct types on the fly. A Struct definition consists of the STRUCT_TYPE byte code, followed be the struct tag (a string uniquely naming the struct), followed by the number of fields in the struct (an integer), followed by the fields

When it sees a struct for the first time, the serialization library assigns it a byte code representation. The first sixteen structs encountered get single byte codes starting at STRUCT_CACHE_PACKED_START. Subsequent structs are encoded as the bytecode STRUCT, plus the ordinal number of the struct's first appearance.

Once a struct has been assigned a byte code representation, that representation is used in subsequent writes instead of the type+tag+fieldcount.

Because the structs encode a fieldcount, naive readers can read structs they have never seen before. If a reader does not have any specific handler for a struct, it can represent the struct via the following logical interface (shown in Java):

public interface Tagged {
    public Object getTag();
    public Object getValue();
    public Map getMeta();


TODO grammar, might use format similar to

should be able to grab from org.fressian.impl.Codes
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