From 4979de888ad699695849a884e0ab8faedd3544f1 Mon Sep 17 00:00:00 2001
Message-Id: <4979de888ad699695849a884e0ab8faedd3544f1.1716598044.git.mdw@distorted.org.uk>
From: Mark Wooding <mdw@chiark.greenend.org.uk>
Date: Tue, 20 Aug 2019 02:31:25 +0100
Subject: [PATCH] src/pset-parse.lisp: Replace `dispatch' by some more
 elementary functions.
Organization: Straylight/Edgeware

From: Mark Wooding <mdw@distorted.org.uk>

This turns out to be an overall saving in terms of lines of code, as
well as being more versatile.  The price is that we've lost the specific
per-operator type mismatch error, but I think that's worth paying.

Somewhat sneakily, I've also fiddled with the operator precedence
numbers, so as to leave more space for other operators, though the
relative precedences are unchanged.
---
 src/pset-parse.lisp | 64 ++++++++++++++++++---------------------------
 1 file changed, 25 insertions(+), 39 deletions(-)

diff --git a/src/pset-parse.lisp b/src/pset-parse.lisp
index e86be27..cfd2b4a 100644
--- a/src/pset-parse.lisp
+++ b/src/pset-parse.lisp
@@ -63,24 +63,21 @@ (defun parse-expression (scanner)
 		      (with-default-error-location (,floc)
 			,@body))))))
 
-    (flet ((dispatch (name args &rest spec)
-	     ;; Evaluate the ARGS to determine their types and values.  Find
-	     ;; the first SPEC, of the form (RETTY OP ARGTY*), where the
-	     ;; ARGTYs match the argument types, in order, and apply OP to
-	     ;; the argument values, return this as a result of type RETTY.
-	     ;; If no SPEC matches, then report an error.
-
-	     (oplambda
-	       (let ((args (mapcar (compose #'funcall #'cons) args)))
-		 (aif (find-if (lambda (item)
-				 (every (lambda (type arg)
-					  (eql type (car arg)))
-					(cddr item) args))
-			       spec)
-		      (values (car it) (apply (cadr it) (mapcar #'cdr args)))
-		      (error "Type mismatch: operator `~A' applied to ~
-			      types ~{~(~A~)~#[~; and ~;, ~]~}"
-			     name (mapcar #'car args)))))))
+    (labels ((want (type thunk)
+	       ;; Evaluate THUNK and convert its result to the given TYPE.
+	       (multiple-value-bind (ty val) (funcall thunk)
+		 (coerce-property-value val ty type)))
+
+	     (int-unop (intop x)
+	       ;; Evaluate X to an integer, and apply INTOP to the result,
+	       ;; giving another integer.
+	       (oplambda (values :int (funcall intop (want :int x)))))
+
+	     (int-binop (intop x y)
+	       ;; Evaluate X and Y to integers, and apply INTOP to the
+	       ;; results, giving another integer.
+	       (oplambda
+		 (values :int (funcall intop (want :int x) (want :int y))))))
 
       (with-parser-context (token-scanner-context :scanner scanner)
 	(when-parse ()
@@ -141,29 +138,18 @@ (defun parse-expression (scanner)
 			       nil nil))))
 
 	      ;; Binary operators.
-	      ((:op #\* binop "*" (x y 7)
-		    (dispatch "*" (list x y) (list :int #'* :int :int)))
-	       (:op #\/ binop "/" (x y 7)
-		    (dispatch "/" (list x y)
-			      (list :int
-				    (lambda (x y)
-				      (cond ((zerop y)
-					     (cerror*
-					      "Division by zero")
-					     (cons :invalid nil))
-					    (t
-					     (floor x y))))
-				    :int :int)))
-	       (:op #\+ binop "+" (x y 5)
-		    (dispatch "+" (list x y) (list :int #'+ :int :int)))
-	       (:op #\- binop "-" (x y 5)
-		    (dispatch "-" (list x y) (list :int #'- :int :int))))
+	      ((:op #\* binop "*" (x y 70) (int-binop #'* x y))
+	       (:op #\/ binop "/" (x y 70)
+		    (oplambda
+		      (let ((x (want :int x)) (y (want :int y)))
+			(when (zerop y) (error "Division by zero"))
+			(values :int (floor x y)))))
+	       (:op #\+ binop "+" (x y 60) (int-binop #'+ x y))
+	       (:op #\- binop "-" (x y 60) (int-binop #'- x y)))
 
 	      ;; Prefix operators.
-	      ((:op #\+ preop "+" (x 9)
-		    (dispatch "+" (list x) (list :int #'+ :int)))
-	       (:op #\- preop "-" (x 9)
-		    (dispatch "-" (list x) (list :int #'- :int)))
+	      ((:op #\+ preop "+" (x 90) (int-unop #'identity x))
+	       (:op #\- preop "-" (x 90) (int-unop #'- x))
 	       (:op #\( lparen #\)))
 
 	      ;; Postfix operators.
-- 
[mdw]