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Type Inference with defineType

The defineType helper is a crucial part of envoyage's type system. It allows you to explicitly define types for some generic parameters while letting TypeScript infer others.

The Problem​

TypeScript has an "all-or-nothing" approach to generic type inference - if you specify one type parameter explicitly, you must specify all of them.

Without defineType, you would need to write:

// Without defineType - must specify ALL type parameters
.addEnv<"local", { env: Record<string, string> }, /* ... */>(
  "local",
  (env) => /* ... */
)

The Solution​

With defineType, you can specify just the types you want while letting TypeScript infer the rest:

.addEnv(
  "local",
  defineType<{ env: Record<string, string> }>(),
  (env) => /* ... */
)

In this example, defineType allows us to:

  1. Explicitly define the environment data structure type: { env: Record<string, string> }
  2. Let TypeScript infer the environment name type from the string literal "local"
  3. Let TypeScript infer the resolution types from the resolution method definitions

The defineType pattern is used throughout envoyage to provide the perfect balance between explicit type definitions where needed and type inference where possible.