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JSON serialization of cty values

The json package allows cty values to be serialized as JSON and decoded back into cty values.

Since the cty type system is a superset of the JSON type system, two modes of operation are possible:

The recommended approach is to define the intended cty data structure as a cty.Type -- possibly involving cty.DynamicPseudoType placeholders -- which then allows full recovery of the original values with correct type information, assuming that the same type description can be provided at decoding time.

Alternatively, this package can decode an arbitrary JSON data structure into the corresponding cty types, which means that it is possible to serialize a cty value without type information and then decode into a value that contains the same data but possibly uses different types to represent that data. This allows direct integration with the standard library encoding/json package, at the expense of type-lossy deserialization.

Type-preserving JSON Serialization

The Marshal and Unmarshal functions together provide for type-preserving serialization and deserialization (respectively) of cty values.

The pattern for using these functions is to define the intended cty type as a cty.Type instance and then pass an identical type as the second argument to both Marshal and Unmarshal. Assuming an identical type is used for both functions, it is guaranteed that values will round-trip through JSON serialization to produce a value of the same type.

The cty.Type passed to Unmarshal is used as a hint to resolve ambiguities in the mapping to JSON. For example, cty list, set and tuple types all lower to JSON arrays, so additional type information is needed to decide which type to use when unmarshaling.

The cty.Type passed to Marshal serves a more subtle purpose. Any cty.DynamicPseudoType placeholders in the type will cause extra type information to be saved in the JSON data structure, which is then used by Unmarshal to recover the original type.

Type-preserving JSON serialization is able to serialize and deserialize capsule-typed values whose encapsulated Go types are JSON-serializable, except when those values are conformed to a cty.DynamicPseudoType. However, since capsule values compare by pointer equality, a decoded value will not be equal (as cty defines it) with the value that produced it.

Type-lossy JSON Serialization

If a given application does not need to exactly preserve the type of a value, the SimpleJSONValue type provides a simpler method for JSON serialization that works with the encoding/json package in Go's standard library.

SimpleJSONValue is a wrapper struct around cty.Value, which can be embedded into another struct used with the standard library Marshal and Unmarshal functions:

type Example struct {
    Name  string          `json:"name"`
    Value SimpleJSONValue `json:"value"`
}

var example Example
example.Name = "Ermintrude"
example.Value = SimpleJSONValue{cty.NumberIntVal(43)}

Since no specific cty type is available when unmarshalling into SimpleJSONValue, a straightforward mapping is used:

  • JSON strings become cty.String values.
  • JSON numbers become cty.Number values.
  • JSON booleans become cty.Bool values.
  • JSON arrays become cty.Tuple-typed values whose element types are selected via this mapping.
  • JSON objects become cty.Object-typed values whose attribute types are selected via this mapping.
  • Any JSON null is mapped to cty.NullVal(cty.DynamicPseudoType).

The above mapping is unambiguous and lossless, so any valid JSON buffer can be decoded into an equally-expressive cty value, but the type may not exactly match that of the value used to produce the JSON buffer in the first place.