fix(maybe): restrict mapped fmap value from being nullable in resul…

…ting type
drizzer14 · Nov 24, 2022 · 95895eb · 95895eb
1 parent 6ee5d6d
commit 95895eb
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Showing 2 changed files with 9 additions and 2 deletions.
diff --git a/src/maybe/operators/fmap.ts b/src/maybe/operators/fmap.ts
@@ -17,7 +17,7 @@ import { isJust } from './guards'
  */
 export default function fmap<Value, NextValue>(
   map: Map<Value, NextValue>
-): (monad: Maybe<Value>) => Maybe<NextValue>
+): (monad: Maybe<Value>) => Maybe<NonNullable<NextValue>>
 
 /**
  * Maps the value of the provided `monad` through the `transition` function
@@ -26,7 +26,7 @@ export default function fmap<Value, NextValue>(
 export default function fmap<Value, NextValue>(
   monad: Maybe<Value>,
   map: Map<Value, NextValue>
-): Maybe<NextValue>
+): Maybe<NonNullable<NextValue>>
 
 export default function fmap (...args: [any, any?]): any {
   return permutation2(

diff --git a/typetests/maybe.test.ts b/typetests/maybe.test.ts
@@ -80,6 +80,13 @@ const c2: Maybe<string> = tacit(
   maybe(1)
 )
 
+const c3: Maybe<string> = fmap(
+  maybe(1),
+  (v) => {
+    return v > 0 ? `${v}` : null
+  }
+)
+
 // endregion
 
 // region fold