Fix mapError exception handling

src/composable/composable.ts

@@ -78,7 +78,7 @@ function pipe<T extends [Composable, ...Composable[]]>(
     //@ts-ignore pipe uses exactly he same generic input type as sequence
     //           I don't understand what is the issue here but ignoring the errors
     //           is safe and much nicer than a bunch of casts to any
-    const res = (await sequence(...fns)(...args))
+    const res = await sequence(...fns)(...args)
     return !res.success ? error(res.errors) : success(res.data.at(-1))
   }) as PipeReturn<T>
 }
@@ -204,7 +204,13 @@ function mapError<T extends Composable, R>(
 ) {
   return (async (...args) => {
     const res = await fn(...args)
-    return !res.success ? error(mapper(res).errors) : success(res.data)
+    if (res.success) return success(res.data)
+    const mapped = await composable(mapper)(res)
+    if (mapped.success) {
+      return error(mapped.data.errors)
+    } else {
+      return error(mapped.errors)
+    }
   }) as T
 }
 

src/composable/index.test.ts

@@ -331,6 +331,21 @@ describe('map', () => {
     assertEquals(res.success, false)
     assertEquals(res.errors![0].message, 'a is 1')
   })
+
+  it('fails when mapper fail', async () => {
+    const fn = map(add, () => {
+      throw new Error('Mapper also has problems')
+    })
+    const res = await fn(1, 2)
+
+    type _FN = Expect<
+      Equal<typeof fn, Composable<(a: number, b: number) => never>>
+    >
+    type _R = Expect<Equal<typeof res, Result<never>>>
+
+    assertEquals(res.success, false)
+    assertEquals(res.errors![0].message, 'Mapper also has problems')
+  })
 })
 
 const cleanError = (err: ErrorWithMessage) => ({
@@ -351,5 +366,20 @@ describe('mapError', () => {
     assertEquals(res.success, false)
     assertEquals(res.errors![0].message, 'a is 1!!!')
   })
+
+  it('fails when mapper fail', async () => {
+    const fn = mapError(faultyAdd, () => {
+      throw new Error('Mapper also has problems')
+    })
+    const res = await fn(1, 2)
+
+    type _FN = Expect<
+      Equal<typeof fn, Composable<(a: number, b: number) => number>>
+    >
+    type _R = Expect<Equal<typeof res, Result<number>>>
+
+    assertEquals(res.success, false)
+    assertEquals(res.errors![0].message, 'Mapper also has problems')
+  })
 })