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Serialization Schema

This document details the existing mappings in NimYAML from Nim types to YAML tags. Throughout this document, there are two tag shorthands being used:


The first one is defined by the YAML specification and is used for types from the YAML failsafe, JSON or core schema. The second one is defined by NimYAML and is used for types from the Nim standard library.

The YAML tag system has no understanding of generics. This means that NimYAML must map every generic type instance to a YAML tag that describes that exact type instance. For example, a seq[string] is mapped to the tag !n!system:seq(;2002:string).

As you can see, the expanded tag handle of the generic type parameter is added to the tag of the generic type. To be compliant with the YAML spec, the following modifications are made:

If a type takes multiple generic parameters, the tag handles are separated by semicolons within the parentheses. Note that this does not guarantee unique tag handles for every type, but it is currently seen as good enough.

Note that user-defined generic types are currently not officially supported by NimYAML. Only the generic collection types explicitly listed here use this mechanism for crafting YAML tags.

Scalar Types

The following table defines all non-composed, atomar types that are mapped to YAML types by NimYAML.

Nim typeYAML tag
string!!string (or !n!nil:string if nil)
int!n!system:int32 (independent on target architecture)
uint!n!system:uint32 (independent from target architecture)

Apart from these standard library types, NimYAML also supports all enum types as scalar types. They will be serialized to their string representation.

Apart from the types listed here and enum tyes, no atomar types are supported.

Collection Types

Collection types in Nim are typically generic. As such, they take their contained types as parameters inside parentheses as explained above. The following types are supported:

Nim typeYAML tagYAML structure
array!n!system:array(?;?) (first parameter like 0..5)sequence
seq!n!system:seq(?) (or !n!nil:seq if nil)sequence
OrderedTable!n!tables:OrderedTable(?;?)sequence of single-pair mappings

Standard YAML Types

NimYAML does not support all types defined in the YAML specification, not even those of the failsafe schema. The reason is that the failsafe schema is designed for dynamic type systems where a sequence can contain arbitrarily typed values. This is not fully translatable into a static type system. NimYAML does support some mechanisms to make working with heterogeneous collection structures easier, see Serialization Overview.

Note that because the specification only defines that an implementation should implement the failsafe schema, NimYAML is still compliant; it has valid reasons not to implement the schema.

This is a full list of all types defined in the YAML specification or the YAML type registry. It gives an overview of which types are supported by NimYAML, which may be supported in the future and which will never be supported.

YAML typeStatus
!!mapCannot be supported
!!omapCannot be supported
!!pairsCannot be supported
!!setCannot be supported
!!seqCannot be supported
!!binaryCurrently not supported
!!boolMaps to Nim's bool type
!!floatNot supported (user can choose)
!!intNot supported (user can choose)
!!mergeNot supported and unlikely to be implemented
!!nullUsed for reference types that are nil
!!strMaps to Nim's string type
!!timestampMaps to Nim's Time type
!!valueNot supported and unlikely to be implemented
!!yamlNot supported and unlikely to be implemented

!!int and !!float are not supported out of the box to let the user choose where to map them (for example, !!int may map to int32 or int64, or the the generic int whose size is platform-dependent). If one wants to use !!intor !!float, the process is to create a distinct type derived from the desired base type and then set its tag using setTagUri.

!!merge and !!value are not supported because the semantics of these types would make a multi-pass loading process necessary and if one takes the tag system seriously, !!merge can only be used with YAML's collection types, which, as explained above, cannot be supported.