From e26362ea1b82c47206161b475bf15c9787ff07ae Mon Sep 17 00:00:00 2001
From: Matthew Fluet <matthew.fluet@gmail.com>
Date: Wed, 22 Nov 2023 11:21:46 -0500
Subject: [PATCH 1/2] New regression test demonstrating bug with
 `TextIO.getInstream`

---
 regression/textio.3.ok  |  0
 regression/textio.3.sml | 43 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+)
 create mode 100644 regression/textio.3.ok
 create mode 100644 regression/textio.3.sml

diff --git a/regression/textio.3.ok b/regression/textio.3.ok
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/regression/textio.3.sml b/regression/textio.3.sml
new file mode 100644
index 0000000000..226b39218d
--- /dev/null
+++ b/regression/textio.3.sml
@@ -0,0 +1,43 @@
+(* See https://github.com/MLton/mlton/issues/535. *)
+
+datatype tree = Leaf of char | Node of tree * tree
+
+fun nextis c cs =
+  case TextIO.StreamIO.input1 (TextIO.getInstream cs) of
+    NONE => false
+  | SOME (c', cs') => c' = c
+
+(* This version works. *)
+(*
+fun nextis c cs =
+  case TextIO.lookahead cs of
+    NONE => false
+  | SOME c' => c' = c
+*)
+
+fun discard c cs =
+  let val c' = valOf (TextIO.input1 cs) in
+    if c = c'
+    then ()
+    else raise Fail ("unexpected character: " ^ Char.toString c' ^ ", expecting: " ^ Char.toString c)
+  end
+
+fun parse cs =
+  case valOf (TextIO.input1 cs) of
+   #"(" =>
+      let
+        fun loop l r =
+          if nextis #")" cs
+          then (discard #")" cs; Node (l, r))
+          else loop (Node (l, r)) (parse cs)
+      in
+        loop (parse cs) (parse cs)
+      end
+  | c => Leaf c
+
+val cs = TextIO.openString "(a(bc)d)"
+
+(* with this it works too *)
+(* val true = nextis #"(" cs *)
+
+val t = (parse cs)

From 3d19b9368bf88c8aab69f26fdf270dd2c2c9f97d Mon Sep 17 00:00:00 2001
From: Matthew Fluet <matthew.fluet@gmail.com>
Date: Wed, 22 Nov 2023 11:22:50 -0500
Subject: [PATCH 2/2] Fix bug in `ImperativeIO.getInstream`

Properly flush the "imperative" ImperativeIO buffer when realizing the
underlying stream via `getInstream`.

Failing to reset the `first` and `last` refs when realizing the
`StreamIO.instream` from an `ImperativeIO.instream` that is in
"imperative" mode allows subsequent `ImperativeIO.input*` functions to
advance the stream with respect to the "imperative" mode but not
advance the `StreamIO.instream`.  After realizing the
`StreamIO.instream` from an `ImpearativeIO.instream`, the stream
should be in "stream" mode and always advance the `StreamIO.instream`.

Fixes MLton/mlton#535
---
 basis-library/io/imperative-io.fun | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/basis-library/io/imperative-io.fun b/basis-library/io/imperative-io.fun
index 231e640b80..41cf3987db 100644
--- a/basis-library/io/imperative-io.fun
+++ b/basis-library/io/imperative-io.fun
@@ -1,4 +1,4 @@
-(* Copyright (C) 2013,2017 Matthew Fluet.
+(* Copyright (C) 2013,2017,2023 Matthew Fluet.
  * Copyright (C) 2002-2007 Henry Cejtin, Matthew Fluet, Suresh
  *    Jagannathan, and Stephen Weeks.
  *
@@ -681,7 +681,7 @@ fun getInstream (ib as In {state, ...}) =
                                        AS.vector (AS.slice (buf, f, 
                                                             SOME (l - f)))))
                         else doit (false, NONE)
-                  val () = state := Stream s
+                  val () = setInstream (ib, s)
                in
                   s
                end