docs: add inference helpers recipe

recipes/inference-helpers.md

@@ -0,0 +1,80 @@
+## Inference Helpers
+
+`@trpc/server` exports the following helper types to assist with inferring these types from the `router` exported in `~/server/trpc/index.ts`:
+
+- `inferProcedureOutput<TProcedure>`
+- `inferProcedureInput<TProcedure>`
+- `inferSubscriptionOutput<TRouter, TPath>`
+
+```ts
+// ~/utils/trpc.ts
+import type { router } from '~/server/trpc/index.ts'
+
+type AppRouter = typeof router
+
+/**
+ * Enum containing all api query paths
+ */
+export type TQuery = keyof AppRouter['_def']['queries']
+
+/**
+ * Enum containing all api mutation paths
+ */
+export type TMutation = keyof AppRouter['_def']['mutations']
+
+/**
+ * Enum containing all api subscription paths
+ */
+export type TSubscription = keyof AppRouter['_def']['subscriptions']
+
+/**
+ * This is a helper method to infer the output of a query resolver
+ * @example type HelloOutput = InferQueryOutput<'hello'>
+ */
+export type InferQueryOutput<TRouteKey extends TQuery> = inferProcedureOutput<
+  AppRouter['_def']['queries'][TRouteKey]
+>
+
+/**
+ * This is a helper method to infer the input of a query resolver
+ * @example type HelloInput = InferQueryInput<'hello'>
+ */
+export type InferQueryInput<TRouteKey extends TQuery> = inferProcedureInput<
+  AppRouter['_def']['queries'][TRouteKey]
+>
+
+/**
+ * This is a helper method to infer the output of a mutation resolver
+ * @example type HelloOutput = InferMutationOutput<'hello'>
+ */
+export type InferMutationOutput<TRouteKey extends TMutation> =
+  inferProcedureOutput<AppRouter['_def']['mutations'][TRouteKey]>
+
+/**
+ * This is a helper method to infer the input of a mutation resolver
+ * @example type HelloInput = InferMutationInput<'hello'>
+ */
+export type InferMutationInput<TRouteKey extends TMutation> =
+  inferProcedureInput<AppRouter['_def']['mutations'][TRouteKey]>
+
+/**
+ * This is a helper method to infer the output of a subscription resolver
+ * @example type HelloOutput = InferSubscriptionOutput<'hello'>
+ */
+export type InferSubscriptionOutput<TRouteKey extends TSubscription> =
+  inferProcedureOutput<AppRouter['_def']['subscriptions'][TRouteKey]>
+
+/**
+ * This is a helper method to infer the asynchronous output of a subscription resolver
+ * @example type HelloAsyncOutput = InferAsyncSubscriptionOutput<'hello'>
+ */
+export type InferAsyncSubscriptionOutput<TRouteKey extends TSubscription> =
+  inferSubscriptionOutput<AppRouter, TRouteKey>
+
+/**
+ * This is a helper method to infer the input of a subscription resolver
+ * @example type HelloInput = InferSubscriptionInput<'hello'>
+ */
+export type InferSubscriptionInput<TRouteKey extends TSubscription> =
+  inferProcedureInput<AppRouter['_def']['subscriptions'][TRouteKey]>
+```