Default decode hooks are not accessible when overriding DecodeHook

[andig]

Preflight Checklist

• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

WithDecodeHook (and the DecodeHook decoder option) replace viper's default hook chain rather than extend it. The default chain is built in defaultDecoderConfig:

decodeHook = mapstructure.ComposeDecodeHookFunc(
	mapstructure.StringToTimeDurationHookFunc(),
	stringToWeakSliceHookFunc(","),
)

stringToWeakSliceHookFunc is unexported, so any application that adds a single custom hook silently loses the weak string-to-slice behaviour and cannot restore it — the only workaround is to copy the function into the application.

The difference is observable: decoding "a,b" into []string yields ["a", "b"] with viper's default chain and ["a,b"] without it (with WeaklyTypedInput).

Proposed Solution

Either:

1. Export the default chain, e.g. viper.DefaultDecodeHook() returning the composed hook (and/or export StringToWeakSliceHookFunc), so callers can compose:

   viper.WithDecodeHook(mapstructure.ComposeDecodeHookFunc(
       myHook,
       viper.DefaultDecodeHook(),
   ))

2. Or add an option that appends to the defaults instead of replacing them, e.g. viper.WithAdditionalDecodeHook(h).

Option 1 is the smaller change and keeps composition explicit.

Alternatives Considered

Copying stringToWeakSliceHookFunc into the application — works, but duplicates viper internals and drifts when the default chain changes (the source already carries a TODO about replacing that implementation).

Additional Information

Viper v1.21.0. Encountered while adding an ${env:NAME} resolution hook to an application's config decoding, where the rest of viper's default decoding behaviour must stay intact.

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🤖 Filed with Claude Code