Add aplist library

aplist.scm

@@ -0,0 +1,58 @@
+;; Copyright 2021 Lassi Kortela
+;; SPDX-License-Identifier: MIT
+
+;;; ->
+
+(define (alist->plist alist)
+  (let loop ((tail alist) (plist '()))
+    (if (null? tail) (reverse plist)
+        (let ((entry (car tail)))
+          (if (pair? entry)
+              (loop (cdr tail)
+                    (cons (cdr entry) (cons (car entry) plist)))
+              (error "Invalid alist" alist))))))
+
+(define (plist->alist plist)
+  (let loop ((tail plist) (alist '()))
+    (if (null? tail) (reverse alist)
+        (if (and (pair? tail) (pair? (cdr tail)))
+            (loop (cddr tail)
+                  (cons (cons (car tail) (cadr tail)) alist))
+            (error "Invalid plist" plist)))))
+
+;;; fold
+
+(define (alist-fold merge state alist)
+  (let loop ((tail alist) (state state))
+    (if (null? tail) state
+        (let ((entry (car tail)))
+          (if (pair? entry)
+              (loop (cdr tail) (merge (car entry) (cdr entry) state))
+              (error "Invalid alist" alist))))))
+
+(define (plist-fold merge state plist)
+  (let loop ((tail plist) (state state))
+    (if (null? tail) state
+        (if (and (pair? tail) (pair? (cdr tail)))
+            (loop (cddr tail) (merge (car tail) (cadr tail) state))
+            (error "Invalid plist" plist)))))
+
+;;; for-each
+
+(define (alist-for-each fn alist)
+  (alist-fold (lambda (key val _) (fn key val) #f)
+              '() alist))
+
+(define (plist-for-each fn plist)
+  (plist-fold (lambda (key val _) (fn key val) #f)
+              '() plist))
+
+;;; map
+
+(define (alist-map fn alist)
+  (reverse (alist-fold (lambda (key val acc) (cons (fn key val) acc))
+                       '() alist)))
+
+(define (plist-map fn plist)
+  (reverse (plist-fold (lambda (key val acc) (cons (fn key val) acc))
+                       '() plist)))

aplist.sld

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+(define-library (alist)
+  (export alist->plist
+          plist->alist
+          alist-fold
+          plist-fold
+          alist-for-each
+          plist-for-each
+          alist-map
+          plist-map)
+  (import (scheme base))
+  (include "aplist.scm"))