Config Merging

Config merging allows composing Teckel documents by deep-merging multiple YAML values. This is useful for layering environment-specific overrides on top of a base configuration.

Module: crates/teckel-parser/src/resolve/config_merger.rs

Merge semantics

The merge follows these rules:

Value type	Behavior
Objects (mappings)	Deep merge -- overlay fields override base fields recursively
Arrays	Replaced entirely by the overlay
Scalars	Replaced by the overlay

API

/// Deep-merge two YAML values.
pub fn deep_merge(base: Value, overlay: Value) -> Value;

/// Merge multiple YAML documents left to right.
/// The rightmost document has the highest precedence.
pub fn merge_documents(documents: Vec<Value>) -> Option<Value>;

Example: Object deep merge

Given a base:

a: 1
b:
  c: 2
  d: 3

And an overlay:

b:
  c: 99
  e: 4

The result is:

a: 1
b:
  c: 99
  d: 3
  e: 4

• a is preserved from the base (not present in overlay)
• b.c is overridden to 99
• b.d is preserved from the base
• b.e is added from the overlay

Example: Array replacement

Arrays are not merged element by element -- the overlay replaces the entire array:

# Base
items:
  - a
  - b

# Overlay
items:
  - x

# Result
items:
  - x

Multi-document merge

merge_documents accepts a vector of YAML values and merges them left to right. The rightmost document has the highest precedence:

use serde_yaml::Value;

let base: Value = serde_yaml::from_str("a: 1\nb: 2")?;
let env: Value = serde_yaml::from_str("b: 99\nc: 3")?;
let local: Value = serde_yaml::from_str("c: 100")?;

let merged = merge_documents(vec![base, env, local]);
// Result: { a: 1, b: 99, c: 100 }

Use cases

• Environment layering: Base config + staging overrides + production overrides
• Shared defaults: Common input/output settings shared across pipelines
• Template composition: Merging template parameters with per-pipeline values