feat: produce helpful error messages on httpRequest codec errors

Attempts to produce a helpful error message when invalid codecs are passed to `httpRequest`.
It is a workaround until something like microsoft/TypeScript#40468 is merged.

Author: bitgopatmcl

packages/io-ts-http/src/httpRequest.ts

@@ -1,7 +1,6 @@
 import * as t from 'io-ts';
 import { Json } from 'io-ts-types';
 import { flattened, optional, optionalized } from './combinators';
-import { OutputConstrainedProps } from './utils';
 
 export const GenericHttpRequest = optionalized({
   // DISCUSS: renaming this to something more specific, e.g. route, or path, or routeParams, or pathParams
@@ -18,13 +17,46 @@ export type HttpRequestCodec<T> = t.Type<
 >;
 
 export type HttpRequestCombinatorProps = {
-  params?: NonNullable<OutputConstrainedProps<string | undefined>>;
-  query?: NonNullable<OutputConstrainedProps<string | string[] | undefined>>;
-  headers?: NonNullable<OutputConstrainedProps<string | undefined>>;
+  params?: NonNullable<t.Props>;
+  query?: NonNullable<t.Props>;
+  headers?: NonNullable<t.Props>;
   body?: NonNullable<t.Props>;
 };
 
-export function httpRequest<Props extends HttpRequestCombinatorProps>(props: Props) {
+/**
+ * Attempts to produce a helpful error message when invalid codecs are passed to `httpRequest`
+ * It is a workaround until something like https://github.com/microsoft/TypeScript/pull/40468
+ * is merged.
+ */
+type EmitOutputTypeErrors<
+  P extends t.Props | undefined,
+  O,
+  OName extends string,
+> = P extends undefined
+  ? P
+  : {
+      [K in keyof P & string]: P[K] extends t.Type<any, O, any>
+        ? P[K]
+        : `Codec's output type is not assignable to ${OName}. Try using one like \`NumberFromString\``;
+    };
+
+type EmitPropsErrors<P extends HttpRequestCombinatorProps> = {
+  params?: EmitOutputTypeErrors<P['params'], string | undefined, 'string | undefined'>;
+  query?: EmitOutputTypeErrors<
+    P['query'],
+    string | string[] | undefined,
+    'string | string[] | undefined'
+  >;
+  headers?: EmitOutputTypeErrors<
+    P['headers'],
+    string | undefined,
+    'string | undefined'
+  >;
+};
+
+export function httpRequest<
+  Props extends HttpRequestCombinatorProps & EmitPropsErrors<Props>,
+>(props: Props) {
   return flattened('httpRequest', {
     query: {},
     params: {},

packages/io-ts-http/src/utils.ts

@@ -26,10 +26,6 @@ export type OptionalizedC<Props extends t.Props> = t.IntersectionC<
   [t.TypeC<RequiredProps<Props>>, t.PartialC<OptionalProps<Props>>]
 >;
 
-export type OutputConstrainedProps<O> = {
-  [K: string]: t.Type<any, O, unknown>;
-};
-
 export type NestedProps = {
   [K: string]: t.Props;
 };