Make List an interface
I don't plan to provide a different implementation, but this gives me
the flexibility. Code using ps.List without an interface smelled
wrong in all the clients. This should have a cleaner feel.
diff --git a/list.go b/list.go
index d285d27..1fc7dc6 100644
--- a/list.go
+++ b/list.go
@@ -1,41 +1,60 @@
package ps
+type List interface {
+ // IsNil returns true if the list is empty
+ IsNil() bool
+
+ // Cons returns a new list with val added onto the head
+ Cons(val Any) List
+
+ // Head returns the first element in the list or panics if the list is empty
+ Head() Any
+
+ // Tail returns the tail of this list or panics if the list is empty
+ Tail() List
+
+ // Size returns the list's length
+ Size() int
+
+ // ForEach executes a callback for each value in the list
+ ForEach(f func(Any))
+
+ // Reverse returns a list with elements in opposite order as this list
+ Reverse() List
+}
+
// Immutable (i.e. persistent) list
-type List struct {
+type list struct {
depth int // the number of nodes after, and including, this one
value Any
- tail *List
+ tail *list
}
// An empty list shared by all lists
-var nilList = &List{}
+var nilList = &list{}
// NewList returns a new, empty list
-func NewList() *List {
+func NewList() List {
return nilList
}
-// IsNil returns true if the list is empty
-func (self *List) IsNil() bool {
+func (self *list) IsNil() bool {
return self == nilList;
}
-// Size returns the list's length
-func (self *List) Size() int {
+func (self *list) Size() int {
return self.depth
}
-// Cons returns a new list with val added onto the head
-func (tail *List) Cons(val Any) *List {
- var list List
- list.depth = tail.depth + 1
- list.value = val
- list.tail = tail
- return &list
+func (tail *list) Cons(val Any) List {
+ var xs list
+ xs.depth = tail.depth + 1
+ xs.value = val
+ xs.tail = tail
+ return &xs
}
-// Head returns the first element in the list or panics if the list is empty
-func (self *List) Head() Any {
+func (self *list) Head() Any {
if self.IsNil() {
panic("Called Head() on an empty list")
}
@@ -43,8 +62,7 @@
return self.value
}
-// Tail returns the tail of this list or panics if the list is empty
-func (self *List) Tail() *List {
+func (self *list) Tail() List {
if self.IsNil() {
panic("Called Tail() on an empty list")
}
@@ -53,7 +71,7 @@
}
// ForEach executes a callback for each value in the list
-func (self *List) ForEach(f func(Any)) {
+func (self *list) ForEach(f func(Any)) {
if self.IsNil() {
return
}
@@ -62,7 +80,7 @@
}
// Reverse returns a list with elements in opposite order as this list
-func (self *List) Reverse() *List {
+func (self *list) Reverse() List {
reversed := NewList()
self.ForEach( func (v Any) { reversed = reversed.Cons(v) })
return reversed
