bug#69739: 30.0.50; `type-of` is not precise enough

[Stefan Monnier]

To make it more concrete, here's a suggested patch.
Comments?  Objections?


        Stefan


>From 7b017522a11f8d8e0bafab85b7032d4e9763aeff Mon Sep 17 00:00:00 2001
From: Stefan Monnier <monnier@iro.umontreal.ca>
Date: Tue, 12 Mar 2024 09:26:24 -0400
Subject: [PATCH] (type-of): Return more precise types (bug#69739)

* src/data.c (Ftype_of): Give more precise types for nil, t, and the
three subcases of `subr`.
(syms_of_data): Define the corresponding new symbols.

* lisp/emacs-lisp/cl-preloaded.el (subr): Demote it to `atom`.
(subr-native-elisp, subr-primitive): Add `compiled-function` as
parent instead.
(special-form): New type.
(finalizer): New (previously missing) type.

* lisp/emacs-lisp/seq.el (seq-remove-at-position): Adjust to `type-of`
returning `null` for nil.

* lisp/obsolete/eieio-compat.el (eieio--generic-static-object-generalizer):
* lisp/emacs-lisp/cl-generic.el (cl--generic-typeof-generalizer): Simplify.

* doc/lispref/objects.texi (Type Predicates): Update accordingly.
---
 doc/lispref/objects.texi        | 13 +++++++------
 etc/NEWS                        |  5 +++++
 lisp/emacs-lisp/cl-generic.el   |  3 +--
 lisp/emacs-lisp/cl-preloaded.el | 11 +++++++----
 lisp/emacs-lisp/seq.el          |  2 +-
 lisp/obsolete/eieio-compat.el   |  2 +-
 src/data.c                      | 14 +++++++++++---
 7 files changed, 33 insertions(+), 17 deletions(-)

diff --git a/doc/lispref/objects.texi b/doc/lispref/objects.texi
index 279f449a994..00581814825 100644
--- a/doc/lispref/objects.texi
+++ b/doc/lispref/objects.texi
@@ -1485,8 +1485,8 @@ Type Descriptors
 @subsection Type Descriptors
 
   A @dfn{type descriptor} is a @code{record} which holds information
-about a type.  Slot 1 in the record must be a symbol naming the type, and
-@code{type-of} relies on this to return the type of @code{record}
+about a type.  The first slot in the record must be a symbol naming the type,
+and @code{type-of} relies on this to return the type of @code{record}
 objects.  No other type descriptor slot is used by Emacs; they are
 free for use by Lisp extensions.
 
@@ -2186,8 +2186,9 @@ Type Predicates
 @code{float}, @code{font-entity}, @code{font-object},
 @code{font-spec}, @code{frame}, @code{hash-table}, @code{integer},
 @code{marker}, @code{mutex}, @code{obarray}, @code{overlay}, @code{process},
-@code{string}, @code{subr}, @code{symbol}, @code{thread},
-@code{vector}, @code{window}, or @code{window-configuration}.
+@code{string}, @code{subr-primitive}, @code{subr-native-elisp},
+@code{special-form}, @code{symbol}, @code{null}, @code{boolean},
+@code{thread}, @code{vector}, @code{window}, or @code{window-configuration}.
 However, if @var{object} is a record, the type specified by its first
 slot is returned; @ref{Records}.
 
@@ -2196,9 +2197,9 @@ Type Predicates
      @result{} integer
 @group
 (type-of 'nil)
-     @result{} symbol
+     @result{} null
 (type-of '())    ; @r{@code{()} is @code{nil}.}
-     @result{} symbol
+     @result{} null
 (type-of '(x))
      @result{} cons
 (type-of (record 'foo))
diff --git a/etc/NEWS b/etc/NEWS
index 19cd170e5c7..d06b3f9c06d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -62,6 +62,11 @@ more details.
 
 * Incompatible Changes in Emacs 30.1
 
+** 'type-of' sometimes returns more precise types.
+More specifically, for nil and t it returns 'null' and 'boolean'
+instead of just 'symbol' and for "subrs", it now returns one of
+'special-form', 'subr-primitive', or 'subr-native-elisp'.
+
 ** Tree-Sitter modes are now declared as submodes of the non-TS modes.
 In order to help the use of those Tree-Sitter modes, they are now
 declared to have the corresponding non-Tree-Sitter mode as an
diff --git a/lisp/emacs-lisp/cl-generic.el b/lisp/emacs-lisp/cl-generic.el
index 84eb800ec24..cc7773ad4a2 100644
--- a/lisp/emacs-lisp/cl-generic.el
+++ b/lisp/emacs-lisp/cl-generic.el
@@ -1339,8 +1339,7 @@ cl--generic-type-specializers
            (cl--class-allparents class)))))
 
 (cl-generic-define-generalizer cl--generic-typeof-generalizer
-  ;; FIXME: We could also change `type-of' to return `null' for nil.
-  10 (lambda (name &rest _) `(if ,name (type-of ,name) 'null))
+  10 #'type-of
   #'cl--generic-type-specializers)
 
 (cl-defmethod cl-generic-generalizers :extra "typeof" (type)
diff --git a/lisp/emacs-lisp/cl-preloaded.el b/lisp/emacs-lisp/cl-preloaded.el
index 5743684fa89..7ab39fed222 100644
--- a/lisp/emacs-lisp/cl-preloaded.el
+++ b/lisp/emacs-lisp/cl-preloaded.el
@@ -355,6 +355,7 @@ frame
 (cl--define-built-in-type buffer atom)
 (cl--define-built-in-type window atom)
 (cl--define-built-in-type process atom)
+(cl--define-built-in-type finalizer atom)
 (cl--define-built-in-type window-configuration atom)
 (cl--define-built-in-type overlay atom)
 (cl--define-built-in-type number-or-marker atom
@@ -415,15 +416,17 @@ compiled-function
   "Abstract type of functions that have been compiled.")
 (cl--define-built-in-type byte-code-function (compiled-function)
   "Type of functions that have been byte-compiled.")
-(cl--define-built-in-type subr (compiled-function)
+(cl--define-built-in-type subr (atom)
   "Abstract type of functions compiled to machine code.")
 (cl--define-built-in-type module-function (function)
   "Type of functions provided via the module API.")
 (cl--define-built-in-type interpreted-function (function)
   "Type of functions that have not been compiled.")
-(cl--define-built-in-type subr-native-elisp (subr)
-  "Type of function that have been compiled by the native compiler.")
-(cl--define-built-in-type subr-primitive (subr)
+(cl--define-built-in-type special-form (subr)
+  "Type of the core syntactic elements of the Emacs Lisp language.")
+(cl--define-built-in-type subr-native-elisp (subr compiled-function)
+  "Type of functions that have been compiled by the native compiler.")
+(cl--define-built-in-type subr-primitive (subr compiled-function)
   "Type of functions hand written in C.")
 
 (unless (cl--class-parents (cl--find-class 'cl-structure-object))
diff --git a/lisp/emacs-lisp/seq.el b/lisp/emacs-lisp/seq.el
index 20077db9e60..cd6f26abdb3 100644
--- a/lisp/emacs-lisp/seq.el
+++ b/lisp/emacs-lisp/seq.el
@@ -363,7 +363,7 @@ seq-remove-at-position
 The result is a sequence of the same type as SEQUENCE."
   (seq-concatenate
    (let ((type (type-of sequence)))
-     (if (eq type 'cons) 'list type))
+     (if (memq type '(null cons)) 'list type))
    (seq-subseq sequence 0 n)
    (seq-subseq sequence (1+ n))))
 
diff --git a/lisp/obsolete/eieio-compat.el b/lisp/obsolete/eieio-compat.el
index 8fdcebbd1c4..c50524d359e 100644
--- a/lisp/obsolete/eieio-compat.el
+++ b/lisp/obsolete/eieio-compat.el
@@ -146,7 +146,7 @@ eieio--generic-static-symbol-generalizer
 (cl-generic-define-generalizer eieio--generic-static-object-generalizer
   ;; Give it a slightly higher priority than `class' so that the
   ;; interleaved list comes before the class's non-interleaved list.
-  51 #'cl--generic-struct-tag
+  51 #'type-of
   (lambda (tag &rest _)
     (and (symbolp tag) (setq tag (cl--find-class tag))
          (eieio--class-p tag)
diff --git a/src/data.c b/src/data.c
index 35f4c82c68f..35bd3b19e45 100644
--- a/src/data.c
+++ b/src/data.c
@@ -202,7 +202,9 @@ DEFUN ("type-of", Ftype_of, Stype_of, 1, 1, 0,
       return Qinteger;
 
     case Lisp_Symbol:
-      return Qsymbol;
+      return NILP (object) ? Qnull
+             : EQ (object, Qt) ? Qboolean
+             : Qsymbol;
 
     case Lisp_String:
       return Qstring;
@@ -224,7 +226,10 @@ DEFUN ("type-of", Ftype_of, Stype_of, 1, 1, 0,
         case PVEC_WINDOW_CONFIGURATION: return Qwindow_configuration;
         case PVEC_PROCESS: return Qprocess;
         case PVEC_WINDOW: return Qwindow;
-        case PVEC_SUBR: return Qsubr;
+        case PVEC_SUBR:
+          return XSUBR (object)->max_args == UNEVALLED ? Qspecial_form
+                 : SUBR_NATIVE_COMPILEDP (object) ? Qsubr_native_elisp
+                 : Qsubr_primitive;
         case PVEC_COMPILED: return Qcompiled_function;
         case PVEC_BUFFER: return Qbuffer;
         case PVEC_CHAR_TABLE: return Qchar_table;
@@ -4202,6 +4207,7 @@ #define PUT_ERROR(sym, tail, msg)                 \
             "Variable binding depth exceeds max-specpdl-size");
 
   /* Types that type-of returns.  */
+  DEFSYM (Qboolean, "boolean");
   DEFSYM (Qinteger, "integer");
   DEFSYM (Qsymbol, "symbol");
   DEFSYM (Qstring, "string");
@@ -4217,7 +4223,9 @@ #define PUT_ERROR(sym, tail, msg)                 \
   DEFSYM (Qwindow_configuration, "window-configuration");
   DEFSYM (Qprocess, "process");
   DEFSYM (Qwindow, "window");
-  DEFSYM (Qsubr, "subr");
+  DEFSYM (Qspecial_form, "special-form");
+  DEFSYM (Qsubr_primitive, "subr-primitive");
+  DEFSYM (Qsubr_native_elisp, "subr-native-elisp");
   DEFSYM (Qcompiled_function, "compiled-function");
   DEFSYM (Qbuffer, "buffer");
   DEFSYM (Qframe, "frame");
-- 
2.43.0