allow custom filtering behavior for Select and MultiSelect (#171)

* Add capability to provide a filter function to filter options.

* Extract DefaultFilterFn to new file.

* Implement custom filter for multiselect as well.

* Add tests.

* Add documentation on custom filtering.

* Improve documentation.

README.md

@@ -153,8 +153,7 @@ prompt := &survey.Select{
 survey.AskOne(prompt, &color, nil)
 ```
 
-The user can filter for options by typing while the prompt is active. The user can also press `esc` to toggle 
-the ability cycle through the options with the j and k keys to do down and up respectively.
+The user can also press `esc` to toggle the ability cycle through the options with the j and k keys to do down and up respectively.
 
 By default, the select prompt is limited to showing 7 options at a time
 and will paginate lists of options longer than that. To increase, you can either
@@ -177,8 +176,7 @@ prompt := &survey.MultiSelect{
 survey.AskOne(prompt, &days, nil)
 ```
 
-The user can filter for options by typing while the prompt is active. The user can also press `esc` to toggle 
-the ability cycle through the options with the j and k keys to do down and up respectively.
+The user can also press `esc` to toggle the ability cycle through the options with the j and k keys to do down and up respectively.
 
 By default, the MultiSelect prompt is limited to showing 7 options at a time
 and will paginate lists of options longer than that. To increase, you can either
@@ -194,6 +192,33 @@ Launches the user's preferred editor (defined by the $EDITOR environment variabl
 temporary file. Once the user exits their editor, the contents of the temporary file are read in as
 the result. If neither of those are present, notepad (on Windows) or vim (Linux or Mac) is used.
 
+## Filtering options in Select and MultiSelect
+
+The user can filter for options by typing while the prompt is active. This will filter out all options that don't contain the 
+typed string anywhere in their name, ignoring case. This default filtering behavior is provided by the `DefaultFilterFn` 
+function.
+
+A custom filter function can also be provided to change this default behavior by providing a value for the `FilterFn` field:
+
+```golang
+&Select{
+    Message: "Choose a color:",
+    Options: []string{"red", "blue", "green"},
+    FilterFn: func(filter string, options []string) (filtered []string) {
+        result := DefaultFilterFn(filter, options)
+        for _, v := range result {
+            if len(v) >= 5 {
+                filtered = append(filtered, v)
+            }
+        }
+        return
+    },
+}
+```
+
+While the example above is contrived, this allows for use cases where "smarter" filtering might be useful, for example, when 
+options are backed by more complex types and filtering might need to occur on more metadata than just the displayed name.
+
 ## Validation
 
 Validating individual responses for a particular question can be done by defining a

filter.go

@@ -0,0 +1,13 @@
+package survey
+
+import "strings"
+
+var DefaultFilterFn = func(filter string, options []string) (answer []string) {
+	filter = strings.ToLower(filter)
+	for _, o := range options {
+		if strings.Contains(strings.ToLower(o), filter) {
+			answer = append(answer, o)
+		}
+	}
+	return answer
+}

multiselect.go

@@ -28,6 +28,7 @@ type MultiSelect struct {
 	PageSize      int
 	VimMode       bool
 	FilterMessage string
+	FilterFn      func(string, []string) []string
 	filter        string
 	selectedIndex int
 	checked       map[string]bool
@@ -146,17 +147,13 @@ func (m *MultiSelect) OnChange(line []rune, pos int, key rune) (newLine []rune,
 }
 
 func (m *MultiSelect) filterOptions() []string {
-	filter := strings.ToLower(m.filter)
-	if filter == "" {
+	if m.filter == "" {
 		return m.Options
 	}
-	answer := []string{}
-	for _, o := range m.Options {
-		if strings.Contains(strings.ToLower(o), filter) {
-			answer = append(answer, o)
-		}
+	if m.FilterFn != nil {
+		return m.FilterFn(m.filter, m.Options)
 	}
-	return answer
+	return DefaultFilterFn(m.filter, m.Options)
 }
 
 func (m *MultiSelect) Prompt() (interface{}, error) {
@@ -166,7 +163,7 @@ func (m *MultiSelect) Prompt() (interface{}, error) {
 	if len(m.Default) > 0 {
 		for _, dflt := range m.Default {
 			for _, opt := range m.Options {
-				// if the option correponds to the default
+				// if the option corresponds to the default
 				if opt == dflt {
 					// we found our initial value
 					m.checked[opt] = true

multiselect_test.go

@@ -8,7 +8,7 @@ import (
 	"strings"
 	"testing"
 
-	expect "github.com/Netflix/go-expect"
+	"github.com/Netflix/go-expect"
 	"github.com/stretchr/testify/assert"
 	"gopkg.in/AlecAivazis/survey.v1/core"
 	"gopkg.in/AlecAivazis/survey.v1/terminal"
@@ -245,6 +245,49 @@ func TestMultiSelectPrompt(t *testing.T) {
 			},
 			[]string{"Tuesday"},
 		},
+		{
+			"Test MultiSelect prompt interaction with filter is case-insensitive",
+			&MultiSelect{
+				Message: "What days do you prefer:",
+				Options: []string{"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"},
+			},
+			func(c *expect.Console) {
+				c.ExpectString("What days do you prefer:  [Use arrows to move, type to filter]")
+				// Filter down to Tuesday.
+				c.Send("tues")
+				// Select Tuesday.
+				c.Send(" ")
+				c.SendLine("")
+				c.ExpectEOF()
+			},
+			[]string{"Tuesday"},
+		},
+		{
+			"Test MultiSelect prompt interaction with custom filter",
+			&MultiSelect{
+				Message: "What days do you prefer:",
+				Options: []string{"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"},
+				FilterFn: func(filter string, options []string) (filtered []string) {
+					result := DefaultFilterFn(filter, options)
+					for _, v := range result {
+						if len(v) >= 7 {
+							filtered = append(filtered, v)
+						}
+					}
+					return
+				},
+			},
+			func(c *expect.Console) {
+				c.ExpectString("What days do you prefer:")
+				// Filter down to days which names are longer than 7 runes
+				c.Send("day")
+				// Select Wednesday.
+				c.Send(string(terminal.KeyArrowDown))
+				c.SendLine(" ")
+				c.ExpectEOF()
+			},
+			[]string{"Wednesday"},
+		},
 		{
 			"Test MultiSelect clears input on select",
 			&MultiSelect{

select.go

@@ -2,8 +2,6 @@ package survey
 
 import (
 	"errors"
-	"strings"
-
 	"gopkg.in/AlecAivazis/survey.v1/core"
 	"gopkg.in/AlecAivazis/survey.v1/terminal"
 )
@@ -28,6 +26,7 @@ type Select struct {
 	PageSize      int
 	VimMode       bool
 	FilterMessage string
+	FilterFn      func(string, []string) []string
 	filter        string
 	selectedIndex int
 	useDefault    bool
@@ -154,17 +153,13 @@ func (s *Select) OnChange(line []rune, pos int, key rune) (newLine []rune, newPo
 }
 
 func (s *Select) filterOptions() []string {
-	filter := strings.ToLower(s.filter)
-	if filter == "" {
+	if s.filter == "" {
 		return s.Options
 	}
-	answer := []string{}
-	for _, o := range s.Options {
-		if strings.Contains(strings.ToLower(o), filter) {
-			answer = append(answer, o)
-		}
+	if s.FilterFn != nil {
+		return s.FilterFn(s.filter, s.Options)
 	}
-	return answer
+	return DefaultFilterFn(s.filter, s.Options)
 }
 
 func (s *Select) Prompt() (interface{}, error) {
@@ -180,7 +175,7 @@ func (s *Select) Prompt() (interface{}, error) {
 	if s.Default != "" {
 		// find the choice
 		for i, opt := range s.Options {
-			// if the option correponds to the default
+			// if the option corresponds to the default
 			if opt == s.Default {
 				// we found our initial value
 				sel = i

select_test.go

@@ -8,7 +8,7 @@ import (
 	"strings"
 	"testing"
 
-	expect "github.com/Netflix/go-expect"
+	"github.com/Netflix/go-expect"
 	"github.com/stretchr/testify/assert"
 	"gopkg.in/AlecAivazis/survey.v1/core"
 	"gopkg.in/AlecAivazis/survey.v1/terminal"
@@ -219,6 +219,22 @@ func TestSelectPrompt(t *testing.T) {
 			},
 			"green",
 		},
+		{
+			"Test Select prompt interaction with filter is case-insensitive",
+			&Select{
+				Message: "Choose a color:",
+				Options: []string{"red", "blue", "green"},
+			},
+			func(c *expect.Console) {
+				c.ExpectString("Choose a color:")
+				// Filter down to red and green.
+				c.Send("RE")
+				// Select green.
+				c.SendLine(string(terminal.KeyArrowDown))
+				c.ExpectEOF()
+			},
+			"green",
+		},
 		{
 			"Can select the first result in a filtered list if there is a default",
 			&Select{
@@ -234,6 +250,29 @@ func TestSelectPrompt(t *testing.T) {
 			},
 			"red",
 		},
+		{
+			"Test Select prompt interaction with custom filter",
+			&Select{
+				Message: "Choose a color:",
+				Options: []string{"red", "blue", "green"},
+				FilterFn: func(filter string, options []string) (filtered []string) {
+					result := DefaultFilterFn(filter, options)
+					for _, v := range result {
+						if len(v) >= 5 {
+							filtered = append(filtered, v)
+						}
+					}
+					return
+				},
+			},
+			func(c *expect.Console) {
+				c.ExpectString("Choose a color:")
+				// Filter down to only green since custom filter only keeps options that are longer than 5 runes
+				c.SendLine("re")
+				c.ExpectEOF()
+			},
+			"green",
+		},
 	}
 
 	for _, test := range tests {