feat(Contract): add mapResult method

README.md

@@ -195,5 +195,19 @@ Aliases: `isUndefined`, `passUndefined`, `isUndef`, `passUndef`, `isVoid`, `pass
 
 Creates a contract which expects `undefined`.
 
+## Using with ADT
+
+```js
+import * as t from 'typed-contracts';
+import { type Either, Right, Left } from 'igogo';
+
+const transform = <T>(
+  result: t.ValidationError | T,
+): Either<t.ValidationError, T> =>
+  result instanceof t.ValidationError ? Left(result) : Right(result);
+
+const string = t.string.mapResult(transform);
+```
+
 [status-url]: https://travis-ci.org/bigslycat/typed-contracts
 [status-img]: https://travis-ci.org/bigslycat/typed-contracts.svg?branch=master

src/Contract.js

@@ -24,6 +24,11 @@ export type Contract<+T> = {
     ...$ReadOnlyArray<void>
   ): (value: mixed) => ValidationError | ?T,
   maybe(valueName: string, value: mixed): ValidationError | ?T,
+
+  mapResult<M>(
+    transform: (result: ValidationError | T) => M,
+  ): ((valueName: string, ...$ReadOnlyArray<void>) => (value: mixed) => M) &
+    ((valueName: string, value: mixed) => M),
 };
 
 declare function curry2<A, B, C>(
@@ -61,8 +66,12 @@ export function of(validate) {
     return result;
   };
 
+  const mapResult = transform =>
+    curry2((valueName, value) => transform(validate(valueName, value)));
+
   contract.maybe = maybe;
   contract.optional = optional;
+  contract.mapResult = mapResult;
 
   return contract;
 }

src/index.d.ts

@@ -53,6 +53,11 @@ export type Contract<T> = {
     valueName: string
   ): (value: unknown) => ValidationError | void | null | T,
   maybe(valueName: string, value: unknown): ValidationError | void | null | T,
+
+  mapResult<M>(
+    transform: (result: ValidationError | T) => M,
+  ): ((valueName: string) => (value: unknown) => M) &
+    ((valueName: string, value: unknown) => M),
 };
 
 export declare function of<T>(