Chapter 3: Fragment Inheritance

In real projects, many configuration blocks look similar - for example, multiple services require region, timeout, dns. Repetitive writing is both verbose and prone to inconsistency. SML uses fragments to solve this problem.

One sentence: Fragment is “value reuse” - defining a set of fields once and expanding them everywhere by referencing.

3.1 Definition fragment: @name

Define a reusable field using @Name { }:

@net {
    region: cn-north-1
    dns: internal.example.com
    timeout: 30
}

Note that @net itself will not appear in the parsing results, it is just a “template”.

3.2 Quoted excerpt: &name

Expand the fragment using &Name:

network: &net

After parsing, network will obtain:

network {
    region: cn-north-1
    dns: internal.example.com
    timeout: 30
}

3.3 References within Blocks

Fragments are commonly used to inject common fields into multiple services:

@base {
    region: cn-north-1
    timeout: 30
}

service auth {
    cfg: &base
    port: 7100
    name: auth-svc
}
service billing {
    cfg: &base
    port: 7200
    name: billing-svc
}

service auth is equivalent to:

service auth {
    cfg: { region: cn-north-1, timeout: 30 }
    port: 7100
    name: auth-svc
}

service billing also obtained the same region/timeout via its cfg key, but port/name were different from each other. Reuse+individuality, perfect.

3.4 Important Details: &name will not be unfolded when written naked within the block

This is the easiest pit for beginners to step into:

server {
    &base            # ❌  Wrong: &base is parsed as a "key name"; fields are NOT expanded
    port: 8080
}

The correct way to write it is to assign a fragment as a value to a key:

server {
    net: &base       # ✅  Key "net" receives all fields of base
    port: 8080
}

Alternatively, &base itself can be used as the source of the value (such as the writing style of service auth { &base ... } in 3.3, which “starts the block directly with &base and then with additional fields”, the parser will treat it as “expanding the fragment first and merging subsequent fields”). * * Remember: Fragments are “values” and should appear at the value position of Key: Value. **

3.5 Fragment vs Contract (Preview)

You may wonder, “Do fragments and contracts (Chapter 5) sound like each other?” Their positioning is completely different:

The two are orthogonal and can be used together (first unfold the fragment, then use contract verification).

3.6 Give it a try with your hands

Write configuration for a game server cluster, with three servers sharing region and max_players:

@common {
    region: ap-east-1
    max_players: 64
}

lobby { &common port: 25565 name: hall }
pvp { &common port: 25566 name: arena }
survival { &common port: 25567 name: survival }

→ Chapter 4: Include and namespaces

Hands on practice

After reading this chapter, directly modify SML in the editor below and click “Run” to immediately see the parsing results or validation errors - having output is necessary for efficient learning.