94 lines
1.9 KiB
Go
94 lines
1.9 KiB
Go
package ps
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// List is a persistent list of possibly heterogenous values.
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type List interface {
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// IsNil returns true if the list is empty
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IsNil() bool
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// Cons returns a new list with val as the head
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Cons(val Any) List
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// Head returns the first element of the list;
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// panics if the list is empty
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Head() Any
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// Tail returns a list with all elements except the head;
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// panics if the list is empty
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Tail() List
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// Size returns the list's length. This takes O(1) time.
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Size() int
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// ForEach executes a callback for each value in the list.
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ForEach(f func(Any))
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// Reverse returns a list whose elements are in the opposite order as
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// the original list.
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Reverse() List
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}
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// Immutable (i.e. persistent) list
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type list struct {
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depth int // the number of nodes after, and including, this one
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value Any
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tail *list
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}
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// An empty list shared by all lists
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var nilList = &list{}
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// NewList returns a new, empty list. The result is a singly linked
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// list implementation. All lists share an empty tail, so allocating
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// empty lists is efficient in time and memory.
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func NewList() List {
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return nilList
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}
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func (self *list) IsNil() bool {
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return self == nilList
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}
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func (self *list) Size() int {
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return self.depth
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}
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func (tail *list) Cons(val Any) List {
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var xs list
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xs.depth = tail.depth + 1
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xs.value = val
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xs.tail = tail
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return &xs
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}
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func (self *list) Head() Any {
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if self.IsNil() {
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panic("Called Head() on an empty list")
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}
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return self.value
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}
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func (self *list) Tail() List {
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if self.IsNil() {
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panic("Called Tail() on an empty list")
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}
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return self.tail
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}
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// ForEach executes a callback for each value in the list
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func (self *list) ForEach(f func(Any)) {
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if self.IsNil() {
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return
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}
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f(self.Head())
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self.Tail().ForEach(f)
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}
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// Reverse returns a list with elements in opposite order as this list
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func (self *list) Reverse() List {
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reversed := NewList()
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self.ForEach(func(v Any) { reversed = reversed.Cons(v) })
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return reversed
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}
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