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Module yaml

Module yaml

Overview

The yaml module takes strings in YAML format and decodes them, or takes a series of non-YAML values and encodes them.

Index

Below is a list of all yaml functions and members.

Name Use
yaml.encode() Convert a Lua object to a YAML string
yaml.decode() Convert a YAML string to a Lua object
__serialize parameter Output structure specification
yaml.cfg() Change configuration
yaml.NULL Analog of Lua’s “nil”
yaml.encode(lua_value)

Convert a Lua object to a YAML string.

Parameters:
  • lua_value – either a scalar value or a Lua table value.
Return:

the original value reformatted as a YAML string.

Rtype:

string

yaml.decode(string)

Convert a YAML string to a Lua object.

Parameters:
  • string – a string formatted as YAML.
Return:

the original contents formatted as a Lua table.

Rtype:

table

__serialize parameter:

The YAML output structure can be specified with __serialize:

  • ‘seq’, ‘sequence’, ‘array’ - table encoded as an array
  • ‘map’, ‘mappping’ - table encoded as a map
  • function - the meta-method called to unpack serializable representation of table, cdata or userdata objects

‘seq’ or ‘map’ also enable the flow (compact) mode for the YAML serializer (flow=”[1,2,3]” vs block=” - 1n - 2n - 3n”).

Serializing ‘A’ and ‘B’ with different __serialize values brings different results:

tarantool> yaml.encode(setmetatable({'A', 'B'}, { __serialize="seq"}))
---
- '["A","B"]'
...
tarantool> yaml.encode(setmetatable({'A', 'B'}, { __serialize="map"}))
---
- '{"1":"A","2":"B"}'
...
tarantool> yaml.encode({setmetatable({f1 = 'A', f2 = 'B'}, { __serialize="map"})})
---
- '[{"f2":"B","f1":"A"}]'
...
tarantool> yaml.encode({setmetatable({f1 = 'A', f2 = 'B'}, { __serialize="seq"})})
---
- '[[]]'
...
yaml.cfg(table)

Set values affecting the behavior of encode and decode functions.

The values are all either integers or boolean true/false.

Option Default Use
cfg.encode_invalid_numbers true A flag saying whether to enable encoding of NaN and Inf numbers
cfg.encode_number_precision 14 Precision of floating point numbers
cfg.encode_load_metatables true A flag saying whether the serializer will follow __serialize metatable field
cfg.encode_use_tostring false A flag saying whether to use tostring() for unknown types
cfg.encode_invalid_as_nil false A flag saying whether to use NULL for non-recognized types
cfg.encode_sparse_convert true A flag saying whether to handle excessively sparse arrays as maps. See detailed description below
cfg.encode_sparse_ratio 2 1/encode_sparse_ratio is the permissible percentage of missing values in a sparse array
cfg.encode_sparse_safe 10 A limit ensuring that small Lua arrays are always encoded as sparse arrays (instead of generating an error or encoding as map)
cfg.decode_invalid_numbers true A flag saying whether to enable decoding of NaN and Inf numbers
cfg.decode_save_metatables true A flag saying whether to set metatables for all arrays and maps

Sparse arrays features:

During encoding, The YAML encoder tries to classify table into one of four kinds:

  • map - at least one table index is not unsigned integer
  • regular array - all array indexes are available
  • sparse array - at least one array index is missing
  • excessively sparse array - the number of values missing exceeds the configured ratio

An array is excessively sparse when all the following conditions are met:

  • encode_sparse_ratio > 0
  • max(table) > encode_sparse_safe
  • max(table) > count(table) * encode_sparse_ratio

The YAML encoder will never consider an array to be excessively sparse when encode_sparse_ratio = 0. The encode_sparse_safe limit ensures that small Lua arrays are always encoded as sparse arrays. By default, attempting to encode an excessively sparse array will generate an error. If encode_sparse_convert is set to true, excessively sparse arrays will be handled as maps.

yaml.cfg() example 1:

The following code will encode 0/0 as NaN (“not a number”) and 1/0 as Inf (“infinity”), rather than returning nil or an error message:

yaml = require('yaml')
yaml.cfg{encode_invalid_numbers = true}
x = 0/0
y = 1/0
yaml.encode({1, x, y, 2})

The result of the yaml.encode() request will look like this:

tarantool> yaml.encode({1, x, y, 2})
---
- '[1,nan,inf,2]
...

yaml.cfg example 2:

To avoid generating errors on attempts to encode unknown data types as userdata/cdata, you can use this code:

tarantool> httpc = require('http.client').new()
---
...

tarantool> yaml.encode(httpc.curl)
---
- error: unsupported Lua type 'userdata'
...

tarantool> yaml.encode(httpc.curl, {encode_use_tostring=true})
---
- '"userdata: 0x010a4ef2a0"'
...

Note

To achieve the same effect for only one call to yaml.encode() (i.e. without changing the configuration permanently), you can use yaml.encode({1, x, y, 2}, {encode_invalid_numbers = true}).

Similar configuration settings exist for JSON and MsgPack.

yaml.NULL

A value comparable to Lua “nil” which may be useful as a placeholder in a tuple.

Example

tarantool> yaml = require('yaml')
---
...
tarantool> y = yaml.encode({'a', 1, 'b', 2})
---
...
tarantool> z = yaml.decode(y)
---
...
tarantool> z[1], z[2], z[3], z[4]
---
- a
- 1
- b
- 2
...
tarantool> if yaml.NULL == nil then print('hi') end
hi
---
...

The YAML collection style can be specified with __serialize:

  • __serialize="sequence" for a Block Sequence array,
  • __serialize="seq" for a Flow Sequence array,
  • __serialize="mapping" for a Block Mapping map,
  • __serialize="map" for a Flow Mapping map.

Serializing ‘A’ and ‘B’ with different __serialize values causes different results:

tarantool> yaml = require('yaml')
---
...

tarantool> yaml.encode(setmetatable({'A', 'B'}, { __serialize="sequence"}))
---
- '---

  - A

  - B

  ...

  '
...

tarantool> yaml.encode(setmetatable({'A', 'B'}, { __serialize="seq"}))
---
- '--- [''A'', ''B'']

  ...

  '
...

tarantool> yaml.encode({setmetatable({f1 = 'A', f2 = 'B'}, { __serialize="map"})})
---
- '---

  - {''f2'': ''B'', ''f1'': ''A''}

  ...

  '
...