Go Specifics

Mapry generates a Go package with structures and several public functions for de/serialization from/to JSONable objects given as interface{}.


In order to generate the Go code, you need to specify the Go specific setting in the schema:


indicates the package name of the generated code.

Generated Code

All the files are generated in a single directory. The code lives in the package indicated by the Go setting package of the schema.

Mapry writes the following files (in order of abstraction):

  • types.go defines all the structures of the object graph (embeddable structures, classes, object graph itself etc.)

  • parse.go defines general parsing structures and their handling (such as parsing errors).

  • fromjsonable.go provides functions for parsing the object graph from a JSONable interface{} value.

  • tojsonable.go gives you functions for serializing the object graph to a JSONable interface{} value.

The example of the generated code for the schema given in Introductory Example is available in the repository.


Assuming the schema provided in Introductory Example, you deserialize the object graph from a JSONable interface{} as follows.

var value interface{}
// ... parse the value from a source, e.g., a file

pipeline := &address.Pipeline{}
errors := address.NewErrors(0)

const referencePath = "/path/to/the/file.json#"


if !errors.Empty() {
    ee := errors.Values()
    for i := 0; i < len(ee); i++ {
            os.Stderr, "%s: %s\n",
            ee[i].Ref, ee[i].Message)
    return 1

The access to the deserialized object graph pipeline is straight-forward:

fmt.Println("Maintainers are:")
for _, m := range pipeline.maintainers {
        "%s (address: %s)\n",
        m.full_name, m.address.text)


Assuming the deserialized pipeline, you serialize it back into a JSONable map[string]interface{}:

var err error
var jsonable map[string]interface{}
jsonable, err = address.PipelineToJSONable(pipeline)

Implementation Details


Go representation of Mapry types tries to be as straight-forward as possible. The following tables show how Mapry types are mapped to Go types in generated Go code.

Primitive types

Mapry type

Go type

















Time zone




Aggregated types (of a generic type T)

Mapry type

Go type





Composite types

Mapry type

Go type

Reference to an instance of class T


Embeddable structure T

struct T

Optional property of type T


Graph-specific structures

Mapry type

Go type

Registry of instances of class T



The standard encoding/json package uses double-precision floating-point numbers (float64) to represent both floating-point and integral numbers. Mapry-generated Go code follows this approach and assumes that all numbers are represented as float64. This assumption has various implications on what numbers can be represented.

The set of representable floating-point numbers equals thus that of float64, namely -1.7976931348623157e+308 to 1.7976931348623157e+308 with the smallest above zero being 2.2250738585072014e-308. Hence Mapry also represents floating points as float64.

Unlike floating-point numbers, which are simply mirroring internal and JSONable representation, Mapry represents integers as int64 which conflicts with JSONable representation of numbers as float64. Namely, according to IEEE 754 standard, float64 use 53 bits to represent digits and 11 bits for the exponent. This means that you can represent all the integers in the range [-2^53, 2^53] (2^53 == 9,007,199,254,740,992) without a loss of precision. However, as you cross 2^53, you lose precision and the set of representable integers becomes sparse. For example, 2^53 + 7 is 9,007,199,254,740,999 while it will be represented as 9,007,199,254,741,000.0 (2^53 + 8) in float64. Hence, you can precisely represent 2^53 + 8, but not 2^53 + 7, in your JSONable.

Unfortunately, most JSON-decoding packages (e.g., encoding/json) will silently ignore this loss of precision. For example, assume you supply a string encoding a JSON object containing an integer property set to 2^53 + 7. You pass this string through encoding/json to obtain a JSONable and then pass it on to Mapry for further parsing. Since Mapry does not directly operate on the string, but on an intermediate JSONable representation (which represents numbers as float64), your Mapry structure ends up with integer representations that diverges from the original string.

Note that this is a practical problem and not merely a theoretical one. For example, unique identifiers are often encoded as 64-bit integers. If they are generated randomly (or use 64-bits to encode extra information etc.) you should represent them in JSON as strings and not numbers. Otherwise, you will get an invalid unique identifier once you decode the JSON.

Furthermore, Mapry representation of integers with 64-bits restricts the range of representable integers to [-2^64, 2^64 - 1] (-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807). In contrast, JSONable representation uses float64 and hence can represent the above-mentioned wider range of float64 (-1.8e+308 to 1.8e+308). Due to this difference in representations, Mapry-generated code will raise an error if a number needs to be parsed into an integer that is out of 64-bit range.

Date/time Format Directives

Go standard package time diverges from many other languages (including C++ and Python) in that it does not support strftime/strptime directives, but a special (american-centered) date/time format of its own (see time.Format). Such format causes a couple of repercussions:

  • First, fractions of seconds are not supported (akin to C/C++ ctime library).

  • Second, certain parts of the format, while unproblematic in strftime directives, cause conflicts in Go. For example, the format "Sun goes up: %Y-%m-%d %H:%M:%S" will be misinterpreted since Sun will be understood as abbreviated weekday in Go. Mapry detects such conflicts as soon as you try to generate Go code and raise an error. However, we leave it to the user to decide to generate code in other languages even though Go code can not be generated.

    Unfortunately, escape codes are not supported in time package and this problem can not be resolved.


Go represents durations as time.Duration which in fact counts the nanoseconds as int64 (see time.Duration).

Mapry will parse the duration strings into time.Duration. Similar to problems in C++ generated code (see Durations in C++), time.Duration can not capture all the strings representable by ISO 8601 period strings. Number of nanoseconds are limited by the range of int64 and can not span periods as short as 300 years (PY300). Furthermore, periods at finer granularity than nanoseconds are impossible to parse either (e.g., PT0.00000000004). If you need to specify such durations, you need to specify the value as string and parse them manually.

Mind that durations in other language might introduce additional constraints. For example, Python represents durations as microseconds (see Durations in Python).