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Numio

YAML to TOML Format — Numio

Converts one YAML document to TOML. Nested mappings become [table.sub] sections, lists of mappings become [[array of tables]], and keys with null (~) values are dropped.

Rewriting a YAML config as TOML when a project moves to pyproject.toml, Cargo, Hugo or Netlify TOML settings.

YAML

TOML

When to use it

  • Rewriting a YAML config as TOML when a project moves to pyproject.toml, Cargo, Hugo or Netlify TOML settings.
  • Checking how a nested YAML structure would look as TOML tables before choosing a config format.

How to use

  1. Paste a single YAML document with a mapping at the top level into the left editor, or use the load-file button to open a UTF-8 file.
  2. Fix any reported YAML error (indentation, multiple documents, or a top-level list).
  3. Check dates, nulls and lists in the TOML, then copy it.

YAML

# docker-compose style
version: "3.9"
services:
  web:
    image: nginx:1.27
    ports:
      - "8080:80"
    environment:
      DEBUG: false
      WORKERS: 4
  db:
    image: postgres:16

TOML

version = "3.9"

[services.web]
image = "nginx:1.27"
ports = [ "8080:80" ]

  [services.web.environment]
  DEBUG = false
  WORKERS = 4

[services.db]
image = "postgres:16"

Details and limitations

YAML is read as YAML 1.2: yes stays the string "yes", 0o17 becomes 15, and unquoted dates like 2024-01-05 become TOML strings, not TOML dates.

Comments and anchors are not kept as such: aliases (*name) and <<: *anchor merge keys are expanded into the values they refer to.

Only one document is accepted; input with --- separators fails with a multiple-documents error. A top-level list, or a list mixing mappings with plain values, is rejected with an explanation.

Runs in your browser (yaml + @iarna/toml). Sub-tables such as [services.web.environment] are indented by two spaces; TOML ignores this indentation.

FAQ

Can I convert a docker-compose.yml to TOML?

The structure converts, as in the example, but Docker Compose only reads YAML. Use this to inspect or reuse the data, not as a Compose file.

What happens to null values?

Keys whose value is ~ or null are omitted because TOML has no null type.

Are YAML anchors and merge keys supported?

Yes. Aliases (*name) are expanded, and <<: *base copies the keys of base into the mapping, with keys written next to it taking precedence. The TOML shows the merged values.

Specifications and references