# HG changeset patch
# User Chris Cannam
# Date 1663174909 -3600
#      Wed Sep 14 18:01:49 2022 +0100
# Node ID fdabfb38085a541f6076cb4f051cdd33c44c10b9
# Parent  0299b00a7d3a6707c2c779228e16bebef9ab7526
Add peeks to accompany pops

diff --git a/persistent-array.sig b/persistent-array.sig
--- a/persistent-array.sig
+++ b/persistent-array.sig
@@ -41,5 +41,6 @@
     val isEmpty : 'a array -> bool
     val append : 'a array * 'a -> 'a array
     val popEnd : 'a array -> 'a array * 'a
+    val peekEnd : 'a array -> 'a
                                 
 end
diff --git a/persistent-array.sml b/persistent-array.sml
--- a/persistent-array.sml
+++ b/persistent-array.sml
@@ -43,13 +43,18 @@
 
     fun popEnd (A { size, trie }) =
         case T.find (trie, size - 0w1) of
-            NONE => raise Size
+            NONE => raise Empty
           | SOME x =>
             let val t = T.remove (trie, size - 0w1)
             in
                 (A { size = size - 0w1, trie = t }, x)
             end
 
+    fun peekEnd (A { size, trie }) =
+        case T.find (trie, size - 0w1) of
+            NONE => raise Empty
+          | SOME x => x
+
     fun sub (v as A { size, trie }, i) =
         if i < 0 orelse i >= length v
         then raise Subscript
diff --git a/persistent-queue.sig b/persistent-queue.sig
--- a/persistent-queue.sig
+++ b/persistent-queue.sig
@@ -25,11 +25,13 @@
     val isEmpty : 'a queue -> bool
     val append : 'a queue * 'a -> 'a queue
     val popEnd : 'a queue -> 'a queue * 'a
+    val peekEnd : 'a queue -> 'a
             
     (* II. Functions specific to PERSISTENT_QUEUE *)
             
     val queue : int * 'a -> 'a queue
     val prepend : 'a queue * 'a -> 'a queue
     val popStart : 'a queue -> 'a queue * 'a
+    val peekStart : 'a queue -> 'a
 
 end
diff --git a/persistent-queue.sml b/persistent-queue.sml
--- a/persistent-queue.sml
+++ b/persistent-queue.sml
@@ -33,12 +33,17 @@
 
     fun popStart ({ start, size, trie } : 'a queue) : ('a queue * 'a) =
         case T.find (trie, start) of
-            NONE => raise Size
+            NONE => raise Empty
           | SOME x =>
             let val t = T.remove (trie, start)
             in
                 ({ start = start + 0w1, size = size - 0w1, trie = t }, x)
             end
+
+    fun peekStart ({ start, size, trie } : 'a queue) : 'a =
+        case T.find (trie, start) of
+            NONE => raise Empty
+          | SOME x => x
             
     fun append ({ start, size, trie }, x) =
         let val _ = if size = maxLenW then raise Size else ()
@@ -49,13 +54,18 @@
 
     fun popEnd ({ start, size, trie } : 'a queue) : ('a queue * 'a) =
         case T.find (trie, start + size - 0w1) of
-            NONE => raise Size
+            NONE => raise Empty
           | SOME x =>
             let val t = T.remove (trie, start + size - 0w1)
             in
                 ({ start = start, size = size - 0w1, trie = t }, x)
             end
 
+    fun peekEnd ({ start, size, trie } : 'a queue) : 'a =
+        case T.find (trie, start + size - 0w1) of
+            NONE => raise Empty
+          | SOME x => x
+
     fun sub (v as { start, size, trie }, i) =
         if i < 0 orelse i >= length v
         then raise Subscript