Skip to main content

Scala JSON-Codecs Backend Documentation

This document provides a description of the JSON codecs backend. The Json Codecs Backend for Scala contains functions to generate Codecs from types in the IR.
Circe is used as the base JSON library.

graph TD;
    Backend-->Core;
    Backend-->Features

The Scala backend is split into two aspects: \

  1. Core - the core Scala codes representing the user business logic
  2. Feature - the Codecs for each type defined in the input model

1. Elm Type to Scala Type Mapping​

This section describes the mapping of Elm types to Scala Codec. It's important to note that all codecs generated from the Elm types are Scala values defined in the Scala AST. The Codec files generated contains value declarations as lamda functions. Below is an outline of how each type is mapped

Variable​

Variable types in Elm are mapped to Scala Variables

Reference​

Reference types in Elm are mapped to Scala Ref which uses the Elm path and name to create a Scala Ref value

Tuple​

Not completed yet

Record​

An Elm record type maps to Circe object which consists of a list of fields which are applied to the Circe.json.obj function.

ExtensibleRecord​

Similar to a record

Function​

Functions are currently not mapped.

Unit​

Functions are currently not mapped.

Custom Type Definition​

Codecs for custom types are composed using helper functions.

2. Scala Json Codecs Backend Functions​

The following functions are defined in the Scala Json Codecs backend

MapModuleDefinitionToCodecs​

This function takes a modules definition and generates the encoders and decoders for the types defined in that module

generateEncodeReference​

Generates and encoder reference for the input type using the FQName

generateDecodeReference​

Generates and decoder reference for the input type using the FQName

mapTypeDefinitionToEncoder​

Type definition could be any of the following:
Type Alias Definition - maps to an encoder for record type
Custom Type Definition - uses helper functions to build encoder for custom types \

mapTypeDefinitionToDecoder​

Type definition could be any of the following:
Type Alias Definition - maps to an encoder for record type
Custom Type Definition - uses helper functions to build encoder for custom types \

mapCustomTypeDefinitionToEncoder​

Builds encoder for custom type using pattern matching

mapCustomTypeDefinitionToDecoder​

Builds a decoder for custom type using pattern mathing on the type constructors.

composeEncoders​

Builds an encoder for one member of a union type

composeDecoders​

Builds a decoder for on member of a union type using for comprehension.

3. Conditionally Generate Codecs​

Codecs can be generated conditionally by adding the --s flag to the morphir-elm gen command.