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Read dotenv

The core reads and writes only YAML. dotenv (.env) comes from a sibling module, config-dotenv, which adds nothing to your dependency graph — a .env file is read with the standard library.

go get gitlab.com/phpboyscout/go/config-dotenv

import (
    "gitlab.com/phpboyscout/go/config"
    configdotenv "gitlab.com/phpboyscout/go/config-dotenv"
)

store, err := config.NewStore(ctx,
    config.WithFiles(fsys, "/etc/app.yaml"),                      // YAML, the default
    config.WithBackend(configdotenv.New(fsys, "/etc/app.env")),   // outranks it
)

Keys nest on underscores

A dotenv key is lower-cased and its underscores become dotted-key separators:

DATABASE_HOST=localhost
DATABASE_PORT=5432

host := store.View().GetString("database.host") // "localhost"
port := store.View().GetInt("database.port")    // 5432

A dotenv layer therefore merges into a nested YAML or JSON one by the same paths. The rule is deterministic — the same mapping the environment backend uses for an unmatched variable — so a key with a literal underscore in its name cannot be expressed, by design. An export prefix is accepted, and surrounding single or double quotes are stripped.

No interpolation

A ${VAR} in a value is left as written, never expanded:

PATH_HINT=${HOME}   # read literally as "${HOME}"

Expanding it would read the process environment through a side door the Store does not own — exactly the unprefixed-environment leak the environment backend's required prefix exists to prevent. If you want environment values, add config.WithEnv("PREFIX") as an ordinary layer, where the prefix applies and provenance names the variable.

Read-only

Values are strings; the read path casts. The format is read-only.

What it costs

Only the config graph — a .env file is parsed with the standard library, so there is no parser module. An allowlist test in the module states it. No filesystem library: you supply the config.FS.