expand on maps delete to handle the case where word doesn't exist (quii#787)

Author: primlock

maps.md

@@ -537,7 +537,7 @@ We get 3 errors this time, but we know how to deal with these.
 const (
 	ErrNotFound         = DictionaryErr("could not find the word you were looking for")
 	ErrWordExists       = DictionaryErr("cannot add word because it already exists")
-	ErrWordDoesNotExist = DictionaryErr("cannot update word because it does not exist")
+	ErrWordDoesNotExist = DictionaryErr("cannot perform operation on word because it does not exist")
 )
 
 func (d Dictionary) Update(word, definition string) error {
@@ -631,9 +631,66 @@ func (d Dictionary) Delete(word string) {
 }
 ```
 
-Go has a built-in function `delete` that works on maps. It takes two arguments. The first is the map and the second is the key to be removed.
+Go has a built-in function `delete` that works on maps. It takes two arguments and returns nothing. The first argument is the map and the second is the key to be removed.
 
-The `delete` function returns nothing, and we based our `Delete` method on the same notion. Since deleting a value that's not there has no effect, unlike our `Update` and `Add` methods, we don't need to complicate the API with errors.
+## Refactor
+There isn't much to refactor, but we can implement the same logic from `Update` to handle cases where word doesn't exist.
+
+```go
+func TestDelete(t *testing.T) {
+	t.Run("existing word", func(t *testing.T) {
+		word := "test"
+		dictionary := Dictionary{word: "test definition"}
+
+		err := dictionary.Delete(word)
+
+		assertError(t, err, nil)
+
+		_, err = dictionary.Search(word)
+
+		assertError(t, err, ErrNotFound)
+	})
+
+	t.Run("non-existing word", func(t *testing.T) {
+		word := "test"
+		dictionary := Dictionary{}
+
+		err := dictionary.Delete(word)
+
+		assertError(t, err, ErrWordDoesNotExist)
+	})
+}
+```
+
+## Try to run test
+
+The compiler will fail because we are not returning a value for `Delete`.
+
+```
+./dictionary_test.go:77:10: dictionary.Delete(word) (no value) used as value
+./dictionary_test.go:90:10: dictionary.Delete(word) (no value) used as value
+```
+
+## Write enough code to make it pass
+
+```go
+func (d Dictionary) Delete(word string) error {
+	_, err := d.Search(word)
+
+	switch err {
+	case ErrNotFound:
+		return ErrWordDoesNotExist
+	case nil:
+		delete(d, word)
+	default:
+		return err
+	}
+
+	return nil
+}
+```
+
+We are again using a switch statement to match on the error when we attempt to delete a word that doesn't exist. 
 
 ## Wrapping up
 

maps/v6/dictionary.go

@@ -7,8 +7,8 @@ const (
 	// ErrWordExists means you are trying to add a word that is already known
 	ErrWordExists = DictionaryErr("cannot add word because it already exists")
 
-	// ErrWordDoesNotExist occurs when trying to update a word not in the dictionary
-	ErrWordDoesNotExist = DictionaryErr("cannot update word because it does not exist")
+	// ErrWordDoesNotExist occurs when trying to perform an operation on a word not in the dictionary
+	ErrWordDoesNotExist = DictionaryErr("cannot perform operation on word because it does not exist")
 )
 
 // DictionaryErr are errors that can happen when interacting with the dictionary.

maps/v7/dictionary.go

@@ -7,8 +7,8 @@ const (
 	// ErrWordExists means you are trying to add a word that is already known
 	ErrWordExists = DictionaryErr("cannot add word because it already exists")
 
-	// ErrWordDoesNotExist occurs when trying to update a word not in the dictionary
-	ErrWordDoesNotExist = DictionaryErr("cannot update word because it does not exist")
+	// ErrWordDoesNotExist occurs when trying to perform an operation on a word not in the dictionary
+	ErrWordDoesNotExist = DictionaryErr("cannot perform operation on word because it does not exist")
 )
 
 // DictionaryErr are errors that can happen when interacting with the dictionary.
@@ -64,6 +64,17 @@ func (d Dictionary) Update(word, definition string) error {
 }
 
 // Delete removes a word from the dictionary.
-func (d Dictionary) Delete(word string) {
-	delete(d, word)
+func (d Dictionary) Delete(word string) error {
+	_, err := d.Search(word)
+	switch err {
+	case ErrNotFound:
+		return ErrWordDoesNotExist
+	case nil:
+		delete(d, word)
+	default:
+		return err
+
+	}
+
+	return nil
 }

maps/v7/dictionary_test.go

@@ -68,13 +68,27 @@ func TestUpdate(t *testing.T) {
 }
 
 func TestDelete(t *testing.T) {
-	word := "test"
-	dictionary := Dictionary{word: "test definition"}
+	t.Run("existing word", func(t *testing.T) {
+		word := "test"
+		dictionary := Dictionary{word: "test definition"}
 
-	dictionary.Delete(word)
+		err := dictionary.Delete(word)
+
+		assertError(t, err, nil)
+
+		_, err = dictionary.Search(word)
+
+		assertError(t, err, ErrNotFound)
+	})
 
-	_, err := dictionary.Search(word)
-	assertError(t, err, ErrNotFound)
+	t.Run("non-existing word", func(t *testing.T) {
+		word := "test"
+		dictionary := Dictionary{}
+
+		err := dictionary.Delete(word)
+
+		assertError(t, err, ErrWordDoesNotExist)
+	})
 }
 
 func assertStrings(t testing.TB, got, want string) {