94 lines
2.8 KiB
EmacsLisp
94 lines
2.8 KiB
EmacsLisp
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;;; stack.el --- Working with stacks in Elisp -*- lexical-binding: t -*-
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;; Author: William Carroll <wpcarro@gmail.com>
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;;; Commentary:
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;; A stack is a LIFO queue.
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;; The design goal here is to expose an intuitive API for working with stacks in
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;; non-mutative way.
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;;
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;; TODO: Consider naming a Functor instance "Mappable."
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;; TODO: Consider naming a Foldable instance "Reduceable."
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;;
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;; TODO: Consider implementing an instance for Mappable.
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;; TODO: Consider implementing an instance for Reduceable.
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;;; Code:
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(require 'list)
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(cl-defstruct stack xs)
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Create
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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(defun stack/new ()
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"Create an empty stack."
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(make-stack :xs '()))
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(defun stack/from-list (xs)
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"Create a new stack from the list, `XS'."
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(list/reduce (stack/new) #'stack/push xs))
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Read
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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(defun stack/peek (xs)
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"Look at the top element of `XS' without popping it off."
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(->> xs
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stack-xs
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list/head))
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Update
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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(defun stack/push (x xs)
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"Push `X' on `XS'."
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(struct/update stack
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xs
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(>> (list/cons x))
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xs))
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;; TODO: How to return something like {(list/head xs), (list/tail xs)} in Elixir
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;; TODO: How to handle popping from empty stacks?
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(defun stack/pop (xs)
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"Return the stack, `XS', without the top element.
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Since I cannot figure out a nice way of return tuples in Elisp, if you want to
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look at the first element, use `stack/peek' before running `stack/pop'."
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(struct/update stack
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xs
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(>> list/tail)
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xs))
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(defun stack/map-top (f xs)
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"Apply F to the top element of XS."
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(->> xs
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stack/pop
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(stack/push (funcall f (stack/peek xs)))))
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Miscellaneous
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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(defun stack/to-list (xs)
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"Return XS as a list.
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The round-trip property of `stack/from-list' and `stack/to-list' should hold."
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(->> xs
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stack-xs
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list/reverse))
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Predicates
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; TODO: Create a macro that wraps `cl-defstruct' that automatically creates
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;; things like `new', `instance?'.
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(defun stack/instance? (xs)
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"Return t if XS is a stack."
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(stack-p xs))
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(provide 'stack)
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;;; stack.el ends here
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