Document functional default field values

[TarantoolBot]

Product: Tarantool
Since: 3.0
Root document:

• https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/format/
• https://www.tarantool.io/en/doc/latest/concepts/data_model/value_store/#constraint-functions

Depends on: #3520

Details

Reference

Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/format/

The format clause contains, for each field, a definition within braces:
{name='...',type='...'[,default=...][,default_func=...]},
where:

• (Optional) The default_func string value specifies the name of a
  function, that is used to generate a default value for the field. If
  default_func is set, the default value is used as the function
  argument. See field default functions.

---

Concepts

Root document: https://www.tarantool.io/en/doc/latest/concepts/data_model/value_store/#constraint-functions

Field default functions

Stored Lua function can be used to generate a default value for the tuple
field. It can take one optional parameter, and must return exactly one
value. To create a field default function, use func.create with function
body. The function must not yield.

Example:

box.schema.func.create('random_point', {
    language = 'Lua',
    body = 'function(param) return math.random(param.min, param.max) end'
})

box.schema.space.create('test')
box.space.test:create_index('pk')
box.space.test:format({
    {name = 'id', type = 'integer'},
    {name = 'latitude', type = 'number',
     default_func = 'random_point', default = {min = -90, max = 90}},
    {name = 'longitude', type = 'number',
     default_func = 'random_point', default = {min = -180, max = 180}}
})

tarantool> math.randomseed(os.time())
---
...

tarantool> box.space.test:insert{1}
---
- [1, 56, 38]
...

Requested by @Gumix in tarantool/tarantool@b055625.

Definition of done

• Reference: document the default_func value in the space format
• Concepts: add Field default functions section