snapshot: infix

Like parenthesize, I'm copying tests over from https://github.com/akkartik/wart
Unlike parenthesize, though, I can't just transliterate the code itself.
Wart was operating on an intermediate AST representation. Here I'm all
the way down to cells. That seemed like a good idea when I embarked, but
now I'm not so sure. Operating with the right AST data structure allowed
me to more easily iterate over the elements of a list. The natural recursion
for cells is not a good fit.

This patch and the next couple is an interesting case study in what makes
Unix so effective. Yes, you have to play computer, and yes it gets verbose
and ugly. But just diff and patch go surprisingly far in helping build a
picture of the state space in my brain.

Then again, there's a steep gradient of skills here. There are people who
can visualize state spaces using diff and patch far better than me, and
people who can't do it as well as me. Nature, nurture, having different
priorities, whatever the reason. Giving some people just the right crutch
excludes others.

6 files changed, 557 insertions, 26 deletions

diff --git a/mu-init.subx b/mu-init.subx
index 438e29e7..26accb8a 100644
--- a/mu-init.subx
+++ b/mu-init.subx
@@ -14,23 +14,21 @@ Entry:
   bd/copy-to-ebp 0/imm32
   #
 #?   (main 0 0 Primary-bus-secondary-drive)
-#?   (set-cursor-position 0 0x40 0x20)
-#?   (test-parenthesize)
-#?   (test-parenthesize-skips-lines-with-initial-parens)
-#?   (test-parenthesize-skips-single-word-lines)
-  # always first run tests
-  (run-tests)
-  (num-test-failures)  # => eax
-  # call main if tests all passed
-  {
-    3d/compare-eax-and 0/imm32
-    75/jump-if-!= break/disp8
-    c7 0/subop/copy *Running-tests? 0/imm32/false
-    (clear-real-screen)
-    c7 0/subop/copy *Real-screen-cursor-x 0/imm32
-    c7 0/subop/copy *Real-screen-cursor-y 0/imm32
-    (main 0 0 Primary-bus-secondary-drive)
-  }
+  (set-cursor-position 0 0x30 2)
+  (test-infix)
+#?   # always first run tests
+#?   (run-tests)
+#?   (num-test-failures)  # => eax
+#?   # call main if tests all passed
+#?   {
+#?     3d/compare-eax-and 0/imm32
+#?     75/jump-if-!= break/disp8
+#?     c7 0/subop/copy *Running-tests? 0/imm32/false
+#?     (clear-real-screen)
+#?     c7 0/subop/copy *Real-screen-cursor-x 0/imm32
+#?     c7 0/subop/copy *Real-screen-cursor-y 0/imm32
+#?     (main 0 0 Primary-bus-secondary-drive)
+#?   }
 
   # hang indefinitely
   {
diff --git a/shell/infix.mu b/shell/infix.mu
index 0d23b3f4..c0b37cb9 100644
--- a/shell/infix.mu
+++ b/shell/infix.mu
@@ -1,6 +1,533 @@
-fn transform-infix _x-ah: (addr handle cell), trace: (addr trace) {
+fn transform-infix x-ah: (addr handle cell), trace: (addr trace) {
   trace-text trace, "infix", "transform infix"
   trace-lower trace
-  trace-text trace, "infix", "todo"
+#?   draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "a:", 2/fg 0/bg
+#?   dump-cell-from-cursor-over-full-screen x-ah, 7/fg 0/bg
+  transform-infix-2 x-ah, trace
   trace-higher trace
 }
+
+# Break any symbols containing operators down in place into s-expressions
+# Transform (... sym op sym ...) greedily in place into (... (op sym sym) ...)
+# Lisp code typed in at the keyboard will never have cycles
+fn transform-infix-2 _x-ah: (addr handle cell), trace: (addr trace) {
+  var x-ah/edi: (addr handle cell) <- copy _x-ah
+  var x/eax: (addr cell) <- lookup *x-ah
+  # trace x-ah {{{
+  {
+    var should-trace?/eax: boolean <- should-trace? trace
+    compare should-trace?, 0/false
+    break-if-=
+    var stream-storage: (stream byte 0x300)
+    var stream/ecx: (addr stream byte) <- address stream-storage
+    var nested-trace-storage: trace
+    var nested-trace/esi: (addr trace) <- address nested-trace-storage
+    initialize-trace nested-trace, 1/only-errors, 0x10/capacity, 0/visible
+    print-cell x-ah, stream, nested-trace
+    trace trace, "infix", stream
+  }
+  # }}}
+  trace-lower trace
+#?   {
+#?     var foo/eax: int <- copy x
+#?     draw-int32-hex-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, foo, 7/fg 0/bg
+#?   }
+#?   dump-cell-from-cursor-over-full-screen x-ah, 5/fg 0/bg
+  # null? return
+  compare x, 0
+  {
+    break-if-!=
+    trace-higher trace
+    trace-text trace, "infix", "=> NULL"
+    return
+  }
+  # nil? return
+  {
+    var nil?/eax: boolean <- nil? x
+    compare nil?, 0/false
+    break-if-=
+    trace-higher trace
+    trace-text trace, "infix", "=> nil"
+    return
+  }
+  var x-type/ecx: (addr int) <- get x, type
+  # symbol? maybe break it down into a pair
+  {
+    compare *x-type, 2/symbol
+    break-if-=
+    tokenize-infix x-ah, trace
+  }
+  # not a pair? return
+#?   draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "a", 4/fg 0/bg
+#?   draw-int32-decimal-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, *x-type, 5/fg 0/bg
+  {
+    compare *x-type, 0/pair
+    break-if-=
+    trace-higher trace
+    # trace "=> " x-ah {{{
+    {
+      var should-trace?/eax: boolean <- should-trace? trace
+      compare should-trace?, 0/false
+      break-if-=
+      var stream-storage: (stream byte 0x300)
+      var stream/ecx: (addr stream byte) <- address stream-storage
+      write stream, "=> "
+      var nested-trace-storage: trace
+      var nested-trace/esi: (addr trace) <- address nested-trace-storage
+      initialize-trace nested-trace, 1/only-errors, 0x10/capacity, 0/visible
+      print-cell x-ah, stream, nested-trace
+      trace trace, "infix", stream
+    }
+    # }}}
+#?     draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "^", 4/fg 0/bg
+    return
+  }
+#?   draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "b", 4/fg 0/bg
+  # singleton operator? unwrap
+  {
+    var first-ah/ecx: (addr handle cell) <- get x, left
+    {
+      var first/eax: (addr cell) <- lookup *first-ah
+      var infix?/eax: boolean <- operator-symbol? first
+      compare infix?, 0/false
+    }
+    break-if-=
+    var rest-ah/eax: (addr handle cell) <- get x, right
+    var rest/eax: (addr cell) <- lookup *rest-ah
+    var rest-nil?/eax: boolean <- nil? rest
+    compare rest-nil?, 0/false
+    break-if-=
+    copy-object first-ah, x-ah
+    trace-higher trace
+    # trace "=> " x-ah {{{
+    {
+      var should-trace?/eax: boolean <- should-trace? trace
+      compare should-trace?, 0/false
+      break-if-=
+      var stream-storage: (stream byte 0x300)
+      var stream/ecx: (addr stream byte) <- address stream-storage
+      write stream, "=> "
+      var nested-trace-storage: trace
+      var nested-trace/esi: (addr trace) <- address nested-trace-storage
+      initialize-trace nested-trace, 1/only-errors, 0x10/capacity, 0/visible
+      print-cell x-ah, stream, nested-trace
+      trace trace, "infix", stream
+    }
+    # }}}
+    return
+  }
+#?   draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "c", 4/fg 0/bg
+  # non-singleton pair. try to "pinch out" infix pattern at root
+  $transform-infix-2:pinch: {
+    # scan past first three elements
+    var first-ah/ecx: (addr handle cell) <- get x, left
+    var rest-ah/esi: (addr handle cell) <- get x, right
+    var rest/eax: (addr cell) <- lookup *rest-ah
+    compare rest, 0
+    break-if-=
+#?     draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "d", 4/fg 0/bg
+    var second-ah/edx: (addr handle cell) <- get rest, left
+    rest-ah <- get rest, right
+    var rest/eax: (addr cell) <- lookup *rest-ah
+    compare rest, 0
+    break-if-=
+#?     draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "e", 4/fg 0/bg
+    var third-ah/ebx: (addr handle cell) <- get rest, left
+    rest-ah <- get rest, right
+    # if second is not an operator, break
+    {
+      var second/eax: (addr cell) <- lookup *second-ah
+      var infix?/eax: boolean <- operator-symbol? second
+      compare infix?, 0/false
+    }
+    break-if-=
+#?     draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "f", 4/fg 0/bg
+    # swap the top 2
+    swap-cells first-ah, second-ah
+    # if there's more than three elements, perform a more complex 'rotation'
+    rest <- lookup *rest-ah
+    var rest-nil?/eax: boolean <- nil? rest
+    compare rest-nil?, 0/false
+    break-if-!=
+#?     draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "g", 4/fg 0/bg
+    # save and clear third->right
+    var saved-rest-h: (handle cell)
+    var saved-rest-ah/eax: (addr handle cell) <- address saved-rest-h
+    copy-object rest-ah, saved-rest-ah
+    nil rest-ah
+    # create new-node out of first..third and rest
+    var result-h: (handle cell)
+    var result-ah/eax: (addr handle cell) <- address result-h
+    new-pair result-ah, *x-ah, saved-rest-h
+    # save
+    copy-object result-ah, x-ah
+  }
+  # recurse after any pinching
+  var x/eax: (addr cell) <- lookup *x-ah  # refresh
+#?   dump-cell-from-cursor-over-full-screen x-ah, 1/fg 0/bg
+  var left-ah/ecx: (addr handle cell) <- get x, left
+#?   draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "x", 1/fg 0/bg
+#?   dump-cell-from-cursor-over-full-screen left-ah, 2/fg 0/bg
+  transform-infix-2 left-ah, trace
+  var right-ah/ecx: (addr handle cell) <- get x, right
+#?   draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "y", 1/fg 0/bg
+#?   dump-cell-from-cursor-over-full-screen right-ah, 3/fg 0/bg
+  transform-infix-2 right-ah, trace
+#?   draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "z", 1/fg 0/bg
+  trace-higher trace
+    # trace "=> " x-ah {{{
+    {
+      var should-trace?/eax: boolean <- should-trace? trace
+      compare should-trace?, 0/false
+      break-if-=
+      var stream-storage: (stream byte 0x300)
+      var stream/ecx: (addr stream byte) <- address stream-storage
+      write stream, "=> "
+      var nested-trace-storage: trace
+      var nested-trace/esi: (addr trace) <- address nested-trace-storage
+      initialize-trace nested-trace, 1/only-errors, 0x10/capacity, 0/visible
+      print-cell x-ah, stream, nested-trace
+      trace trace, "infix", stream
+    }
+    # }}}
+}
+
+fn swap-cells a-ah: (addr handle cell), b-ah: (addr handle cell) {
+  var tmp-h: (handle cell)
+  var tmp-ah/eax: (addr handle cell) <- address tmp-h
+  copy-object a-ah, tmp-ah
+  copy-object b-ah, a-ah
+  copy-object tmp-ah, b-ah
+}
+
+fn tokenize-infix _sym-ah: (addr handle cell), trace: (addr trace) {
+#?     # non-symbol operators
+#?     {
+#?       var operator?/eax: boolean <- operator-grapheme? g
+#?       compare operator?, 0/false
+#?       break-if-=
+#?       next-operator-token in, out, trace
+#?       break $next-token:case
+#?     }
+}
+
+fn test-infix {
+  check-infix "abc", "abc", "F - test-infix/regular-symbol"
+  check-infix "-3", "-3", "F - test-infix/negative-integer-literal"
+  check-infix "[a b+c]", "[a b+c]", "F - test-infix/string-literal"
+  check-infix "$a", "$a", "F - test-infix/dollar-var"
+  check-infix "$+", "$+", "F - test-infix/dollar-operator"
+  check-infix "(+)", "+", "F - test-infix/operator-without-args"
+  check-infix "(= (+) 3)", "(= + 3)", "F - test-infix/operator-without-args-2"
+  check-infix "($+)", "$+", "F - test-infix/dollar-operator-without-args"
+  check-infix "',(a + b)", "',(+ a b)", "F - test-infix/nested-quotes"
+#?   check-infix "',(+)", "',+", "F - test-infix/nested-quotes-2"
+  check-infix "(a + b)", "(+ a b)", "F - test-infix/simple-list"
+  check-infix "(a (+) b)", "(a + b)", "F - test-infix/wrapped-operator"
+  check-infix "(+ a b)", "(+ a b)", "F - test-infix/prefix-operator"
+  check-infix "(a . b)", "(a . b)", "F - test-infix/dot-operator"
+  check-infix "(a b . c)", "(a b . c)", "F - test-infix/dotted-list"
+  check-infix "(+ . b)", "(+ . b)", "F - test-infix/dotted-list-with-operator"
+  check-infix "(+ a)", "(+ a)", "F - test-infix/unary-operator"
+  check-infix "((a + b))", "((+ a b))", "F - test-infix/nested-list"
+  check-infix "(do (a + b))", "(do (+ a b))", "F - test-infix/nested-list-2"
+  check-infix "(a = (a + 1))", "(= a (+ a 1))", "F - test-infix/nested-list-3"
+#?   check-infix "(a + b + c)", "(+ (+ a b) c)", "F - test-infix/left-associative"
+#?   check-infix "(f a + b)", "(f (+ a b))", "F - test-infix/higher-precedence-than-call"
+#?   check-infix "(f a + b c + d)", "(f (+ a b) (+ c d))", "F - test-infix/multiple"
+#?   check-infix "+a", "(+ a)", "F - test-infix/unary-operator-2"
+#?   check-infix "-a", "(- a)", "F - test-infix/unary-operator-3"
+#?   check-infix "a+b", "(+ a b)", "F - test-infix/no-spaces"
+#?   check-infix "',a+b", "',(+ a b)", "F - test-infix/no-spaces-with-nested-quotes"
+#?   check-infix "$a+b", "(+ $a b)", "F - test-infix/no-spaces-2"
+#?   check-infix "-a+b", "(+ (- a) b)", "F - test-infix/unary-over-binary"
+#?   check-infix "~a+b", "(+ (~ a) b)", "F - test-infix/unary-complement"
+#?   check-infix "(n * n-1)", "(* n (- n 1))", "F - test-infix/no-spaces-over-spaces"
+#?   check-infix "`(a + b)", "`(+ a b)", "F - test-infix/backquote"
+#?   check-infix ",@a+b", ",@(+ a b)", "F - test-infix/unquote-splice"
+#?   check-infix ",@(a + b)", ",@(+ a b)", "F - test-infix/unquote-splice-2"
+}
+
+# helpers
+
+# assumes symbol? is already fully tokenized,
+# consists entirely of either operator or non-operator graphemes
+fn operator-symbol? _x: (addr cell) -> _/eax: boolean {
+  var x/esi: (addr cell) <- copy _x
+  {
+    var x-type/eax: (addr int) <- get x, type
+    compare *x-type, 2/symbol
+    break-if-=
+    return 0/false
+  }
+  var x-data-ah/eax: (addr handle stream byte) <- get x, text-data
+  var _x-data/eax: (addr stream byte) <- lookup *x-data-ah
+  var x-data/esi: (addr stream byte) <- copy _x-data
+  rewind-stream x-data
+  var g/eax: grapheme <- read-grapheme x-data
+  # special case: '$' is reserved for gensyms, and can work with either
+  # operator or non-operator symbols.
+  {
+    compare g, 0x24/dollar
+    break-if-!=
+    {
+      var all-dollars?/eax: boolean <- stream-empty? x-data
+      compare all-dollars?, 0/false
+      break-if-=
+      # '$', '$$', '$$$', etc. are regular symbols
+      return 0/false
+    }
+    g <- read-grapheme x-data
+    loop
+  }
+  var result/eax: boolean <- operator-grapheme? g
+  return result
+}
+
+fn non-operator-grapheme? g: grapheme -> _/eax: boolean {
+  ## whitespace
+  compare g, 9/tab
+  {
+    break-if-!=
+    return 0/false
+  }
+  compare g, 0xa/newline
+  {
+    break-if-!=
+    return 0/false
+  }
+  compare g, 0x20/space
+  {
+    break-if-!=
+    return 0/false
+  }
+  ## we don't really use double quotes
+  compare g, 0x22/double-quote
+  {
+    break-if-!=
+    return 1/true
+  }
+  ## brackets
+  compare g, 0x28/open-paren
+  {
+    break-if-!=
+    return 0/false
+  }
+  compare g, 0x29/close-paren
+  {
+    break-if-!=
+    return 0/false
+  }
+  compare g, 0x5b/open-square-bracket
+  {
+    break-if-!=
+    return 0/false
+  }
+  compare g, 0x5d/close-square-bracket
+  {
+    break-if-!=
+    return 0/false
+  }
+  compare g, 0x7b/open-curly-bracket
+  {
+    break-if-!=
+    return 0/false
+  }
+  compare g, 0x7d/close-curly-bracket
+  {
+    break-if-!=
+    return 0/false
+  }
+  # quotes and unquotes are like symbols for this purpose
+  compare g, 0x27/single-quote
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x60/backquote
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x2c/comma
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x40/at-sign
+  {
+    break-if-!=
+    return 1/true
+  }
+  # - other punctuation
+  compare g, 0x23/hash
+  {
+    break-if-!=
+    return 0/false
+  }
+  return 1/true
+}
+
+fn operator-grapheme? g: grapheme -> _/eax: boolean {
+  # '$' is a symbol char
+  compare g, 0x25/percent
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x26/ampersand
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x27/single-quote
+  {
+    break-if-!=
+    return 0/false
+  }
+  compare g, 0x60/backquote
+  {
+    break-if-!=
+    return 0/false
+  }
+  compare g, 0x2c/comma
+  {
+    break-if-!=
+    return 0/false
+  }
+  compare g, 0x40/at-sign
+  {
+    break-if-!=
+    return 0/false
+  }
+  compare g, 0x2a/asterisk
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x2b/plus
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x2d/dash  # '-' not allowed in symbols
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x2e/period
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x2f/slash
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x3a/colon
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x3b/semi-colon
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x3c/less-than
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x3d/equal
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x3e/greater-than
+  {
+    break-if-!=
+    return 1/true
+  }
+  # '?' is a symbol char
+  compare g, 0x5c/backslash
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x5e/caret
+  {
+    break-if-!=
+    return 1/true
+  }
+  # '_' is a symbol char
+  compare g, 0x7c/vertical-line
+  {
+    break-if-!=
+    return 1/true
+  }
+  compare g, 0x7e/tilde
+  {
+    break-if-!=
+    return 1/true
+  }
+  return 0/false
+}
+
+# helpers for tests
+
+fn check-infix actual: (addr array byte), expected: (addr array byte), message: (addr array byte) {
+  var trace-storage: trace
+  var trace/edx: (addr trace) <- address trace-storage
+#?   initialize-trace trace, 1/only-errors, 0x10/capacity, 0/visible
+  initialize-trace trace, 0x10/levels, 0x1000/capacity, 0/visible
+  #
+  var actual-buffer-storage: gap-buffer
+  var actual-buffer/eax: (addr gap-buffer) <- address actual-buffer-storage
+  initialize-gap-buffer-with actual-buffer, actual
+  var actual-tree-h: (handle cell)
+  var actual-tree-ah/esi: (addr handle cell) <- address actual-tree-h
+  read-cell actual-buffer, actual-tree-ah, trace
+#?   dump-trace-with-label trace, "infix"
+  dump-cell-from-cursor-over-full-screen actual-tree-ah, 7/fg 0/bg
+  var _actual-tree/eax: (addr cell) <- lookup *actual-tree-ah
+  var actual-tree/esi: (addr cell) <- copy _actual-tree
+  #
+  var expected-buffer-storage: gap-buffer
+  var expected-buffer/eax: (addr gap-buffer) <- address expected-buffer-storage
+  initialize-gap-buffer-with expected-buffer, expected
+  var expected-tree-h: (handle cell)
+  var expected-tree-ah/edi: (addr handle cell) <- address expected-tree-h
+  read-without-infix expected-buffer, expected-tree-ah, trace
+  var expected-tree/eax: (addr cell) <- lookup *expected-tree-ah
+  #
+  var match?/eax: boolean <- cell-isomorphic? actual-tree, expected-tree, trace
+  check match?, message
+}
+
+fn read-without-infix in: (addr gap-buffer), out: (addr handle cell), trace: (addr trace) {
+  # eagerly tokenize everything so that the phases are easier to see in the trace
+  var tokens-storage: (stream token 0x400)
+  var tokens/edx: (addr stream token) <- address tokens-storage
+  tokenize in, tokens, trace
+  var error?/eax: boolean <- has-errors? trace
+  compare error?, 0/false
+  {
+    break-if-=
+    dump-trace trace
+    return
+  }
+  # insert more parens based on indentation
+  var parenthesized-tokens-storage: (stream token 0x400)
+  var parenthesized-tokens/ecx: (addr stream token) <- address parenthesized-tokens-storage
+  parenthesize tokens, parenthesized-tokens, trace
+  var error?/eax: boolean <- has-errors? trace
+  compare error?, 0/false
+  {
+    break-if-=
+    dump-trace trace
+    return
+  }
+  parse-input parenthesized-tokens, out, trace
+}
diff --git a/shell/macroexpand.mu b/shell/macroexpand.mu
index 0f3837c7..74f0e9c5 100644
--- a/shell/macroexpand.mu
+++ b/shell/macroexpand.mu
@@ -600,7 +600,7 @@ fn pending-test-macroexpand-inside-nested-backquote-unquote {
   var dummy/eax: boolean <- macroexpand-iter result-ah, globals, trace
   var error?/eax: boolean <- has-errors? trace
   check-not error?, "F - test-macroexpand-inside-nested-backquote-unquote/error"
-  dump-cell-from-cursor-over-full-screen result-ah
+#?   dump-cell-from-cursor-over-full-screen result-ah
   var _result/eax: (addr cell) <- lookup *result-ah
   var result/edi: (addr cell) <- copy _result
   # expected
@@ -610,7 +610,7 @@ fn pending-test-macroexpand-inside-nested-backquote-unquote {
   var expected-h: (handle cell)
   var expected-ah/edx: (addr handle cell) <- address expected-h
   read-cell expected-gap, expected-ah, trace
-  dump-cell-from-cursor-over-full-screen expected-ah
+#?   dump-cell-from-cursor-over-full-screen expected-ah
   var expected/eax: (addr cell) <- lookup *expected-ah
   #
   var assertion/eax: boolean <- cell-isomorphic? result, expected, trace
diff --git a/shell/parse.mu b/shell/parse.mu
index c9e2901f..d888f56b 100644
--- a/shell/parse.mu
+++ b/shell/parse.mu
@@ -8,8 +8,8 @@ fn parse-input tokens: (addr stream token), out: (addr handle cell), trace: (add
     return
   }
   var close-paren?/eax: boolean <- copy 0/false
-  var dummy?/ecx: boolean <- copy 0/false
-  close-paren?, dummy? <- parse-sexpression tokens, out, trace
+  var dot?/ecx: boolean <- copy 0/false
+  close-paren?, dot? <- parse-sexpression tokens, out, trace
   {
     compare close-paren?, 0/false
     break-if-=
@@ -17,6 +17,12 @@ fn parse-input tokens: (addr stream token), out: (addr handle cell), trace: (add
     return
   }
   {
+    compare dot?, 0/false
+    break-if-=
+    error trace, "'.' is not a valid expression"
+    return
+  }
+  {
     var empty?/eax: boolean <- stream-empty? tokens
     compare empty?, 0/false
     break-if-!=
diff --git a/shell/primitives.mu b/shell/primitives.mu
index 1b25583c..44852c8a 100644
--- a/shell/primitives.mu
+++ b/shell/primitives.mu
@@ -1135,7 +1135,7 @@ fn apply-debug _args-ah: (addr handle cell), out: (addr handle cell), trace: (ad
   }
   # dump args->left uglily to screen and wait for a keypress
   var first-ah/eax: (addr handle cell) <- get args, left
-  dump-cell-from-cursor-over-full-screen first-ah
+  dump-cell-from-cursor-over-full-screen first-ah, 7/fg 0/bg
   {
     var foo/eax: byte <- read-key 0/keyboard
     compare foo, 0
diff --git a/shell/print.mu b/shell/print.mu
index ea5db3cf..1f652fa5 100644
--- a/shell/print.mu
+++ b/shell/print.mu
@@ -133,14 +133,14 @@ fn dump-cell-at-top-right in-ah: (addr handle cell) {
   d1, d2 <- draw-stream-wrapping-right-then-down 0/screen, stream, 0/xmin, 0/ymin, 0x80/xmax, 0x30/ymax, 0/x, 0/y, 7/fg, 0xc5/bg=blue-bg
 }
 
-fn dump-cell-from-cursor-over-full-screen in-ah: (addr handle cell) {
+fn dump-cell-from-cursor-over-full-screen in-ah: (addr handle cell), fg: int, bg: int {
   var stream-storage: (stream byte 0x200)
   var stream/edx: (addr stream byte) <- address stream-storage
   var trace-storage: trace
   var trace/edi: (addr trace) <- address trace-storage
   initialize-trace trace, 1/only-errors, 0x10/capacity, 0/visible
   print-cell in-ah, stream, trace
-  draw-stream-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, stream, 7/fg, 0/bg
+  draw-stream-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, stream, fg, bg
 }
 
 fn print-symbol _in: (addr cell), out: (addr stream byte), trace: (addr trace) {