update

4 files changed, 521 insertions, 212 deletions

diff --git a/html/notes/format.md b/html/notes/format.md
index 39da84a..f757736 100644
--- a/html/notes/format.md
+++ b/html/notes/format.md
@@ -1,3 +1,5 @@
+# I hate 'display'
+
 WIP WIP WIP
 
 (format "template" arg ...) ;sprintf
diff --git a/src/libzisp.zig b/src/libzisp.zig
index 174cd40..29b87dd 100644
--- a/src/libzisp.zig
+++ b/src/libzisp.zig
@@ -271,4 +271,6 @@ test "read2" {
 
     const f, const fl = value.sstr.unpack(value.pair.cdr(cdr));
     try std.testing.expectEqualStrings("foo", f[0..fl]);
+
+    _ = read.read("(\"foo\" '\"bar\" [#x \"baz\"])");
 }
diff --git a/src/libzisp/list.zig b/src/libzisp/list.zig
new file mode 100644
index 0000000..a4ce7a8
--- /dev/null
+++ b/src/libzisp/list.zig
@@ -0,0 +1,20 @@
+const value = @import("value.zig");
+
+const Value = value.Value;
+
+pub fn reverse(list: Value) Value {
+    return reverseWithTail(list, value.nil.nil);
+}
+
+pub fn reverseWithTail(list: Value, tail: Value) Value {
+    var head = list;
+    var result = tail;
+    while (!value.nil.check(head)) {
+        value.pair.assert(head);
+        const car = value.pair.car(head);
+        const cdr = value.pair.cdr(head);
+        result = value.pair.cons(car, result);
+        head = cdr;
+    }
+    return result;
+}
diff --git a/src/libzisp/read.zig b/src/libzisp/read.zig
index 812b7c7..74ae51b 100644
--- a/src/libzisp/read.zig
+++ b/src/libzisp/read.zig
@@ -1,21 +1,121 @@
+//
+// === Parser for Code & Data ===
+//
+// Zisp s-expressions come in two flavors: code (sugar) and data (no sugar).
+//
+// Code expressions are first parsed into the same data types which the data
+// expressions can express; it's merely surface-level syntax sugar.
+//
+// Data expressions don't support any syntax sugar and have a simple format,
+// only being able to represent the following data types:
+//
+//   string -> foo , "foo bar"
+//
+//   number -> 123 , -1.23 , +123 , +nan.0 , -inf.0 , ...
+//
+//   rune   -> #foo     ;limited to 6 ASCII letters (a - z, A - Z)
+//
+//   list   -> (...)    ;the usual: actually pairs; may be improper
+//
+//   null   -> ()       ;also called nil around here
+//
+// We may use terms like "code parser" and "data parser" out of convenience,
+// although there may only be a single parser that implements both formats by
+// switching between modes.
+//
+// When the code parser encounters syntax sugar, it always transforms it into a
+// list starting with a rune, like in the following examples:
+//
+//   #(...)   -> (#hash ...)
+//
+//   [...]    -> (#square ...)
+//
+//   'foo     -> (#quote . foo)
+//
+// These can combine:
+//
+//   #{...}   -> (#hash #brace ...)
+//
+//   #'foo    -> (#hash #quote . foo)
+//
+//   ##'[...] -> (#hash #hash #quote #square ...)
+//
+// As a specialty, double-quoted strings are actually considered sugar by the
+// code parser, and are transformed as follows into data:
+//
+//   "..."    -> (#string "...")
+//
+// (Otherwise, all string literals would be identifiers, or all identifiers
+// would be string literals, because Zisp doesn't differentiate strings and
+// symbols like traditional lisps.)
+//
+//
+// === Decoder ===
+//
+// A separate process called "decoding" can transform simple data structures,
+// consisting of only the above types, into a richer set of Zisp data types.
+//
+// For example, the decoder may turn (#hash ...) into a vector.  Some runes may
+// be decoded in isolation rather than as part of a list, which is how #true and #false are implemented.
+//
+//It also
+// interprets common rune invocations like (#quote ...) to implement the
+// traditional quoting mechanism, but in a better way:
+//
+// Traditional quote is "unhygienic" in Scheme terms.  An expressoin such as
+// '(foo bar) will always be read as (quote (foo bar)) regardless of what sort
+// of lexical context it appears in, so the semantics will depend on whatever
+// the identifier "quote" is bound to in that lexical context.
+//
+// The Zisp decoder, which transforms not text to text, but objects to objects,
+// can turn (#quote ...) into an abstract object which encapsulates the notion
+// of quoting, which the Zisp evaluator can recognize and act upon.
+//
+// One way to think about this, in Scheme (R6RS / syntax-case) terms, is that
+// expressions like '(foo bar) turn directly into a *syntax object* when read,
+// rather than a regular list object.
+//
+//
+// === Implementation details ===
+//
+// Instead of using recursion directly, the parser is written in something akin
+// to a manual continuation-passing style, which ensures that parsing depth is
+// not limited by the stack size.
+//
+// All state/context is passed around via a struct pointer.  The parser has a
+// main loop which calls a function, passes it the state, and expects to get a
+// new state pointer in return, which tells which function the main loop should
+// call next, based on the .next field of the state.
+//
+// When a called function wants to call the parser recursively, it sets the
+// .next field to an enumeration value that indicates where the parser should
+// return to after it's done with the sub-parsing, and then constructs a new
+// state struct, saving a pointer to the original in a .parent field.
+//
+// Making the parser "return" is a matter of setting the .retval field, and
+// setting the .next field to the value .finish, to indicate to the main loop
+// that it should either pass control back to the parent, or finish parsing.
+//
+//
+// === Notation used in comments ===
+//
+// Some comments throughout the file give you an example of where the parser
+// currently is in a stream.  These illustrations use the pipe symbol, which
+// looks like a cursor, to indicate the current position of the parser:
+//
+//   (foo| bar baz)   <- parser arrived at the end of the string foo
+//
+//   (foo bar baz)|   <- parser reached EOF (assuming no trailing spaces)
+//
+
 const std = @import("std");
 
 const gc = @import("gc.zig");
+const list = @import("list.zig");
 const value = @import("value.zig");
 
 const Value = value.Value;
 
-const Next = enum {
-    start,
-    datum,
-    hash_end,
-    rune_datum_end,
-    quote_end,
-    list,
-    list_end,
-    done,
-};
-
 const State = struct {
     alloc: std.mem.Allocator,
 
@@ -24,12 +124,14 @@ const State = struct {
 
     mode: enum { code, data } = .code,
 
-    next: Next = .start,
+    next: Next = .start_parsing,
 
     parent: ?*State = null,
 
-    last_rune: ?Value = null,
-    list_tail: ?Value = null,
+    datum_rune: Value = value.boole.f,
+    list_tail: Value = value.nil.nil,
+    opening_bracket: enum { paren, square, brace } = .paren,
+    opening_quote: enum { apos, tick, comma } = .apos,
 
     retval: Value = value.eof.eof,
 
@@ -41,146 +143,303 @@ const State = struct {
         return self.input[self.pos];
     }
 
+    fn skip(self: *State) void {
+        self.pos += 1;
+    }
+
     fn getc(self: *State) u8 {
         const c = self.peek();
-        self.pos += 1;
+        self.skip();
         return c;
     }
 
+    // Consumes whitespace and line comments.
+    fn consumeBlanks(self: *State) void {
+        while (!self.eof()) {
+            if (self.isWhitespace()) {
+                self.skip();
+            } else if (self.peek() == ';') {
+                self.skip();
+                self.consumeLineComment();
+            } else {
+                return;
+            }
+        }
+    }
+
+    fn consumeLineComment(self: *State) void {
+        while (!self.eof()) {
+            if (self.getc() == '\n') {
+                return;
+            }
+        }
+    }
+
+    fn isWhitespace(self: *State) bool {
+        return switch (self.peek()) {
+            '\t', '\n', ' ' => true,
+            else => false,
+        };
+    }
+
+    // Checks for: Whitespace, closing brackets, and EOF.
+    //
+    // This can tell us that we're in a position such as:
+    //
+    //   (foo| bar)
+    //
+    //   (foo bar|)
+    //
+    //   foo|
+    //
+    // We could also accept semicolon, so the following works, like in Scheme:
+    //
+    //   (foo;comment
+    //    bar)
+    //
+    // But IMO this should be an error.  It's too easy to misread, and might
+    // just be a typo: the semicolon may have been meant to be a colon.
+    //
+    fn isEndDelimiter(self: *State) bool {
+        return switch (self.peek()) {
+            '\t', '\n', ' ', ')', ']', '}' => true,
+            else => false,
+        };
+    }
+
     fn isFinalNull(self: *State) bool {
         return self.peek() == 0 and self.pos == self.input.len - 1;
     }
 
-    fn newChild(self: *State, next: Next) *State {
-        const s = self.alloc.create(State) catch @panic("OOM");
-        s.* = State{ .alloc = self.alloc, .input = self.input };
-        s.pos = self.pos;
-        s.mode = self.mode;
-        s.next = next;
-        s.parent = self;
-        return s;
+    fn newSubstate(self: *State, next: Next) *State {
+        const sub = self.alloc.create(State) catch @panic("OOM");
+        sub.* = .{ .alloc = self.alloc, .input = self.input };
+        sub.pos = self.pos;
+        sub.mode = self.mode;
+        sub.next = next;
+        sub.parent = self;
+        return sub;
     }
 
     fn setReturn(self: *State, val: Value) *State {
         self.retval = val;
-        self.next = .done;
+        self.next = .finish;
         return self;
     }
 
     fn finish(self: *State) ?*State {
-        if (self.parent) |p| {
-            p.retval = self.retval;
-            p.pos = self.pos;
-            p.alloc.destroy(self);
-            return p;
+        if (self.parent) |parent| {
+            parent.retval = self.retval;
+            parent.pos = self.pos;
+            parent.alloc.destroy(self);
+            return parent;
         } else {
             return null;
         }
     }
 };
 
+// Probably best *not* to use function pointers here, but rather dispatch to
+// functions manually based on enum value.  This should help the optimizer.
+
+const Next = enum {
+    start_parsing,
+    start_datum,
+    end_hash_datum,
+    end_rune_datum,
+    end_quote,
+    continue_list,
+    end_improper_list,
+    finish,
+};
+
 pub fn read(input: []const u8) Value {
     var gpa: std.heap.GeneralPurposeAllocator(.{}) = .init;
     var top = State{ .alloc = gpa.allocator(), .input = input };
     var s = &top;
-    while (true) s = switch (s.next) {
-        .start => start(s),
-        .datum => datum(s),
-        .hash_end => hashEnd(s),
-        .rune_datum_end => runeDatumEnd(s),
-        .quote_end => quoteEnd(s),
-        .list => list(s),
-        .list_end => list(s),
-        .done => s.finish() orelse break,
-    };
+    while (true) {
+        s = switch (s.next) {
+            .start_parsing => startParsing(s),
+            .start_datum => startDatum(s),
+            .end_hash_datum => endHashDatum(s),
+            .end_rune_datum => endRuneDatum(s),
+            .end_quote => endQuote(s),
+            .continue_list => continueList(s),
+            .end_improper_list => endImproperList(s),
+            .finish => s.finish() orelse break,
+        };
+        std.debug.print("next: {}\n", .{s.next});
+    }
     if (s.eof() or s.isFinalNull()) {
         return s.retval;
     } else {
-        @panic("unconsumed input");
+        // Should never happen.
+        err(s, "READER BUG: unconsumed input");
     }
 }
 
-fn start(s: *State) *State {
-    while (true) {
-        if (s.eof()) {
-            s.next = .done;
-            return s;
-        }
-        const c = s.getc();
-        if (isWhitespace(c)) {
-            continue;
-        }
-        return switch (c) {
-            // whitespace checked above
-            0...31, 127...255 => err(s, "invalid character"),
-            ')', ']', '}' => err(s, "unexpected closing bracket"),
-            ';' => semi(s),
-            else => ret: {
-                // backtrack; let other function handle it
-                s.pos -= 1;
-                break :ret datum(s);
-            },
-        };
+fn startParsing(s: *State) *State {
+    s.consumeBlanks();
+    if (s.eof()) {
+        return s.setReturn(value.eof.eof);
     }
+    return switch (s.peek()) {
+        // whitespace already consumed
+        0...31, 127...255 => err(s, "invalid character"),
+        ')', ']', '}' => err(s, "unexpected closing bracket"),
+        else => startDatum(s),
+    };
 }
 
-fn semi(s: *State) *State {
-    while (true) {
-        if (s.eof()) {
-            s.next = .done;
-            return s;
-        }
-        const c = s.getc();
-        if (c == '\n') {
-            break;
-        }
-    }
-    return s;
-}
-
-fn datum(s: *State) *State {
-    const c = s.getc();
-    if (isWhitespace(c)) {
+// This is called when we *immediately* expect a datum and nothing else; for
+// example, no whitespace or comments, because they've already been consumed.
+fn startDatum(s: *State) *State {
+    if (s.isWhitespace()) {
         return err(s, "expected datum, got whitespace");
     }
-    return switch (c) {
+    if (s.eof()) {
+        return err(s, "expected datum, got EOF");
+    }
+    return switch (s.getc()) {
         // whitespace checked above
         0...31, 127...255 => err(s, "invalid character"),
+
         ')', ']', '}' => err(s, "unexpected closing bracket"),
+
         ';' => err(s, "expected datum, got semicolon"),
-        '"' => string(s),
-        '#' => hash(s),
-        '\'' => quote(s),
-        '(' => list(s),
-        '+' => plus(s),
-        ',' => comma(s),
-        '.' => dot(s),
-        '0'...'9' => number(s),
-        '[' => square(s),
-        '`' => backtick(s),
-        '{' => brace(s),
-        else => symbol(s),
+
+        '#' => handleHash(s),
+
+        '"' => startQuotedString(s),
+
+        '\'', '`', ',' => |c| startQuote(s, c),
+
+        '(', '[', '{' => |c| startList(s, c),
+
+        '+', '-' => |c| handlePlusMinus(s, c),
+
+        '0'...'9' => |c| handleDigit(s, c),
+
+        // Periods only allowed between digits, and to express improper lists.
+        // Things like the following look too much like it could be a typo:
+        //
+        //   (foo .5) (foo .bar)
+        //
+        '.' => err(s, "misplaced period"),
+
+        else => startBareString(s),
     };
 }
 
-fn isWhitespace(c: u8) bool {
-    return switch (c) {
-        '\t', '\n', ' ' => true,
-        else => false,
-    };
+fn handleHash(s: *State) *State {
+    //
+    // We just consumed a hash.  Possibilities include:
+    //
+    //   #|foo       ;rune
+    //
+    //   #|;DATUM    ;datum comment
+    //
+    //   #|DATUM     ;hash-datum (code mode only)
+    //
+
+    if (s.isWhitespace()) {
+        return err(s, "whitespace after hash");
+    }
+    if (s.eof()) {
+        return err(s, "EOF after hash");
+    }
+
+    // Is it a rune?  #foo
+    switch (s.peek()) {
+        'A'...'Z', 'a'...'z' => return handleRune(s),
+        else => {},
+    }
+
+    // Is it a datum comment?  #;DATUM
+    if (s.peek() == ';') {
+        s.skip();
+        // Don't change s.next in this case.  Just let the parser try to redo
+        // what it was doing as soon as the commented-out datum has been read.
+        return s.newSubstate(.start_datum);
+    }
+
+    // Otherwise, it must be a hash-datum.  #DATUM
+
+    // But data mode doesn't allow that.
+    if (s.mode == .data) {
+        return err(s, "invalid use of hash in data mode");
+    }
+
+    s.next = .end_hash_datum;
+    return s.newSubstate(.start_datum);
 }
 
-// Whitespace, semicolon, and closing brackets of any kind
-fn isEndDelimiter(c: u8) bool {
-    return switch (c) {
-        '\t', '\n', ' ', ';' => true,
-        ')', ']', '}' => true,
-        else => false,
+fn handleRune(s: *State) *State {
+    //
+    // We are now here, knowing that at least one ASCII letter follows:
+    //
+    //   #|foo
+    //
+
+    var buf: [6]u8 = undefined;
+    var i: u8 = 0;
+    while (!s.eof()) : (i += 1) switch (s.peek()) {
+        'a'...'z', 'A'...'Z' => {
+            if (i == buf.len) {
+                return err(s, "rune too long");
+            }
+            buf[i] = s.getc();
+        },
+        else => break,
     };
+
+    // Note: 'i' can't be 0 since this function is only called if at least one
+    // ASCII letter is found after the hash.
+
+    const rune = value.rune.pack(buf[0..i]);
+
+    //
+    // Now we're at the end of the rune, but it could be a rune-datum:
+    //
+    //   #foo|(...)
+    //
+
+    if (s.isEndDelimiter()) {
+        return s.setReturn(rune);
+    }
+
+    // Otherwise, it's followed by a datum, like: #foo(...)
+
+    // Which is only allowed in code mode.
+    if (s.mode == .data) {
+        return err(s, "invalid use of hash in data mode");
+    }
+
+    s.datum_rune = rune;
+    s.next = .end_rune_datum;
+    return s.newSubstate(.start_datum);
 }
 
-fn string(s: *State) *State {
-    const str = readString(s) catch return err(s, "unclosed string");
+fn endRuneDatum(s: *State) *State {
+    return s.setReturn(value.pair.cons(
+        s.datum_rune,
+        s.retval,
+    ));
+}
+
+fn endHashDatum(s: *State) *State {
+    return s.setReturn(value.pair.cons(
+        value.rune.pack("hash"),
+        s.retval,
+    ));
+}
+
+fn startQuotedString(s: *State) *State {
+    // We are now here:
+    //
+    //   "|..."
+    //
+    const str = readQuotedString(s) catch return err(s, "unclosed string");
     if (s.mode == .code) {
         // "foo bar" => (#string . "foo bar")
         const rune = value.rune.pack("string");
@@ -193,16 +452,16 @@ fn string(s: *State) *State {
 
 const StringReadError = enum { UnclosedString };
 
-fn readString(s: *State) error{UnclosedString}!Value {
-    return try tryReadSstr(s) orelse readLongString(s);
+fn readQuotedString(s: *State) error{UnclosedString}!Value {
+    return try readQuotedSstr(s) orelse readQuotedLongString(s);
 }
 
-fn tryReadSstr(s: *State) error{UnclosedString}!?Value {
+fn readQuotedSstr(s: *State) error{UnclosedString}!?Value {
     // We will reset to this position if we fail.
     const start_pos = s.pos;
 
     var buf: [6]u8 = undefined;
-    var i: usize = 0;
+    var i: u8 = 0;
     while (!s.eof()) {
         const c = s.getc();
         if (c == '"') {
@@ -221,153 +480,179 @@ fn tryReadSstr(s: *State) error{UnclosedString}!?Value {
     return error.UnclosedString;
 }
 
-fn readLongString(s: *State) Value {
+fn readQuotedLongString(s: *State) Value {
     _ = s;
     @panic("not implemented");
 }
 
-fn hash(s: *State) *State {
-    if (isWhitespace(s.peek())) {
-        return err(s, "whitespace after hash sign");
-    }
-
-    // is it a datum comment?
-    if (s.peek() == ';') {
-        // consume semicolon
-        _ = s.getc();
-        // Just ignore value and return to starting state after reading it.
-        s.next = .start;
-    } else {
-        s.next = .hash_end;
+fn startQuote(s: *State, c: u8) *State {
+    // Allowed in Scheme, but why?  Not in Zisp.
+    if (s.isWhitespace()) {
+        return err(s, "whitespace after apostrophe");
     }
+    s.opening_quote = switch (c) {
+        '\'' => .apos,
+        '`' => .tick,
+        ',' => .comma,
+        else => unreachable,
+    };
+    const sub = s.newSubstate(.start_datum);
+    sub.mode = .data;
+    s.next = .end_quote;
+    return sub;
+}
 
-    // No whitespace or anything; hash must be immediately followed by datum,
-    // including if it's a datum comment.  Note that if it's actually a rune
-    // we're reading, like #foo, we abuse our ability to reading an sstr here
-    // and later turn it into a rune instead, since they're the same length.
-    return s.newChild(.datum);
+fn endQuote(s: *State) *State {
+    const name = switch (s.opening_quote) {
+        .apos => "apos",
+        .tick => "tick",
+        .comma => "comma",
+    };
+    return s.setReturn(value.pair.cons(
+        value.rune.pack(name),
+        s.retval,
+    ));
 }
 
-fn hashEnd(s: *State) *State {
-    // It's not actually a sstr but a rune, like: #foo or #foo(...)
-    if (value.sstr.check(s.retval)) {
-        return hashRuneEnd(s);
+fn startList(s: *State, open: u8) *State {
+    if (s.mode == .data and open != '(') {
+        return err(s, "invalid opening bracket in data mode");
     }
 
-    // Hash followed by an actual datum; becomes a (#hash ...) invocation:
-    //
-    //   #(...) -> (#hash . (...))
-    //
-    //   #"..." -> (#hash . "...")
-    //
+    s.consumeBlanks();
 
-    // But data mode doesn't allow that.
-    if (s.mode == .data) {
-        return err(s, "invalid use of hash in data mode");
+    // Check for empty lists: (), [], {}
+    if (open == '(' and s.peek() == ')') {
+        s.skip();
+        return s.setReturn(value.nil.nil);
     }
-
-    // Also, bare long strings are not OK here; too similar to a rune.
-    if (value.ptr.checkZisp(s.retval, .string)) {
-        return err(s, "long string after hash sign");
+    if (open == '[' and s.peek() == ']') {
+        s.skip();
+        return s.setReturn(value.pair.cons(
+            value.rune.pack("square"),
+            value.nil.nil,
+        ));
+    }
+    if (open == '{' and s.peek() == '}') {
+        s.skip();
+        return s.setReturn(value.pair.cons(
+            value.rune.pack("brace"),
+            value.nil.nil,
+        ));
     }
 
-    return s.setReturn(value.pair.cons(
-        value.rune.pack("hash"),
-        s.retval,
-    ));
+    s.opening_bracket = switch (open) {
+        '(' => .paren,
+        '[' => .square,
+        '{' => .brace,
+        else => unreachable,
+    };
+    s.next = .continue_list;
+    return s.newSubstate(.start_datum);
 }
 
-// Note: Can only come here from hashEnd().
-fn hashRuneEnd(s: *State) *State {
-    // Convert the fake sstr that was meant to be a rune.
-    const rune = value.rune.make(s.retval);
+fn continueList(s: *State) *State {
+    s.consumeBlanks();
 
-    // Maybe it's a stand-alone rune, like: #foo
-    if (isEndDelimiter(s.peek())) {
-        // Which is only allowed in data mode.
-        if (s.mode == .code) {
-            return err(s, "bare runes not allowed in code");
-        } else {
-            return s.setReturn(rune);
-        }
+    if (s.eof()) {
+        return err(s, "unexpected EOF while parsing list");
     }
 
-    // Otherwise, it's followed by a datum, like: #foo(...)
+    const tail = value.pair.cons(s.retval, s.list_tail);
 
-    // Which is only allowed in code mode.
-    if (s.mode == .data) {
-        return err(s, "invalid use of hash in data mode");
-    } else {
-        s.last_rune = rune;
-        s.next = .rune_datum_end;
-        return s.newChild(.datum);
-    }
-}
+    const open = s.opening_bracket;
+    const char = s.peek();
 
-fn runeDatumEnd(s: *State) *State {
-    if (s.last_rune) |rune| {
-        return s.setReturn(value.pair.cons(rune, s.retval));
-    } else {
-        unreachable;
+    // Check for proper ending: (foo bar baz)
+    if (open == .paren and char == ')') {
+        s.skip();
+        return s.setReturn(list.reverse(tail));
+    }
+    if (open == .square and char == ']') {
+        s.skip();
+        return s.setReturn(value.pair.cons(
+            value.rune.pack("square"),
+            list.reverse(tail),
+        ));
+    }
+    if (open == .brace and char == '}') {
+        s.skip();
+        return s.setReturn(value.pair.cons(
+            value.rune.pack("brace"),
+            list.reverse(tail),
+        ));
     }
-}
 
-fn quote(s: *State) *State {
-    // Allowed in Scheme, but why? Not in Zisp.
-    if (isWhitespace(s.peek())) {
-        return err(s, "whitespace after apostrophe");
+    s.list_tail = tail;
+
+    // Check for improper ending: (foo bar . baz)
+    if (char == '.') {
+        s.skip();
+
+        // We should now be at (... foo .| bar) and whitespace must follow.
+        // Scheme allows (foo .(bar)) but we don't.  Mind your spaces!
+        if (!s.isWhitespace()) {
+            return err(s, "invalid use of period");
+        }
+
+        s.consumeBlanks();
+
+        s.next = .end_improper_list;
+        return s.newSubstate(.start_datum);
     }
-    s.next = .quote_end;
-    const c = s.newChild(.datum);
-    c.mode = .data;
-    return c;
-}
 
-fn quoteEnd(s: *State) *State {
-    return s.setReturn(value.pair.cons(
-        value.rune.pack("quote"),
-        s.retval,
-    ));
+    // OK, next element.
+    return s.newSubstate(.start_datum);
 }
 
-fn list(s: *State) *State {
-    return s;
-}
+fn endImproperList(s: *State) *State {
+    s.consumeBlanks();
 
-fn plus(s: *State) *State {
-    return s;
-}
+    if (s.eof()) {
+        return err(s, "unexpected EOF");
+    }
 
-fn comma(s: *State) *State {
-    return s;
-}
+    const open = s.opening_bracket;
+    const char = s.getc();
 
-fn dot(s: *State) *State {
-    return s;
-}
+    const body = s.list_tail;
+    const tail = s.retval;
 
-fn number(s: *State) *State {
-    return s;
-}
+    if (open == .paren and char == ')') {
+        return s.setReturn(list.reverseWithTail(body, tail));
+    }
+    if (open == .square and char == ']') {
+        return s.setReturn(value.pair.cons(
+            value.rune.pack("square"),
+            list.reverseWithTail(body, tail),
+        ));
+    }
+    if (open == .brace and char == '}') {
+        return s.setReturn(value.pair.cons(
+            value.rune.pack("brace"),
+            list.reverseWithTail(body, tail),
+        ));
+    }
 
-fn square(s: *State) *State {
-    return s;
+    return err(s, "malformed list or extra datum at end of improper list");
 }
 
-fn backtick(s: *State) *State {
+fn handlePlusMinus(s: *State, c: u8) *State {
+    _ = c;
     return s;
 }
 
-fn brace(s: *State) *State {
+fn handleDigit(s: *State, c: u8) *State {
+    _ = c;
     return s;
 }
 
-fn symbol(s: *State) *State {
+fn startBareString(s: *State) *State {
     return s;
 }
 
-fn err(s: *State, msg: []const u8) *State {
-    _ = s;
+fn err(s: *State, msg: []const u8) noreturn {
     std.debug.print("{s}\n", .{msg});
+    std.debug.print("pos: {}\n", .{s.pos});
     @panic("reader error");
 }