Prompter as a narrow consumer interface

byob-prompter.1 contextinterfacesprompter

Problem: a wide prompter interface (AskOne, AskMany, 20 options per call) couples every command to a specific library's question shape. When the library changes — or the implementation swaps for tests — the blast radius is large.

Idea: five methods is the minimum that covers a real CLI. Each takes context.Context as its first argument so prompts inherit the caller's deadline and cancellation — a command with a deadline should be able to abandon a hung prompt instead of waiting on stdin forever:

type Prompter interface {
    Confirm(ctx context.Context, msg string, def bool) (bool, error)
    Input(ctx context.Context, msg, def string) (string, error)
    Password(ctx context.Context, msg string) (string, error)
    Select(ctx context.Context, msg string, options []string) (int, error)
    MultiSelect(ctx context.Context, msg string, options []string) ([]int, error)
}

Per byob-interfaces.1, the interface lives in pkg/cmd/prompt/ (or closer to the consumer) and concrete impls (live, stub) satisfy it structurally. No library type leaks into the consumer.

Tradeoffs: five methods won't cover every fancy UX (autocomplete pickers, path completers, multi-line editors). When you need one, add a dedicated method — don't stretch Input to do it. The ctx parameter is uniform across all methods even though most callers will pass the command's root ctx untouched — the consistency matters more than per-method ergonomics.

Design

// pkg/cmd/prompt/prompt.go
package prompt

type Prompter interface {
    Confirm(ctx context.Context, msg string, def bool) (bool, error)
    Input(ctx context.Context, msg, def string) (string, error)
    Password(ctx context.Context, msg string) (string, error)
    Select(ctx context.Context, msg string, options []string) (int, error)
    MultiSelect(ctx context.Context, msg string, options []string) ([]int, error)
}

// Factory holds it as an eager field (cheap, like IOStreams):
type Factory struct {
    IOStreams *iostreams.IOStreams
    Prompter  prompt.Prompter
    // ...lazy fields
}

Commands accept prompt.Prompter as an Options field, not a library type. Tests pass prompt.Stub{...}; production passes prompt.NewLive(io).