[Port `check-arg' from scsh cvs
clinton@unknownlamer.org**20100328214034
 Ignore-this: c0a200ca1c559be841026318f8189db6
] addfile ./module/scsh/utilities.scm
hunk ./module/scsh/utilities.scm 1
-
+;;; Random useful utilities.
+;;; Excerpted from SCSH
+;;; Copyright (c) 1993 by Olin Shivers.
+
+(define-module (scsh utilities)
+  #:export (check-arg))
+
+;;; From guile-scsh/loophole.scm
+;;; Copyright (c) 1993, 1994 Richard Kelsey and Jonathan Rees.  See file COPYING.
+;;; original file: alt/loophole.scm.
+(define-syntax loophole
+  (syntax-rules ()
+    ((loophole ?type ?form)
+     (begin (lambda () ?type)    ;Elicit unbound-variable warnings, etc.
+	    ?form))))
+
+;;; We loophole the call to ERROR -- the point is that perhaps the
+;;; user will interact with a breakpoint, and proceed with a new
+;;; value, which we will then pass to a new invocation of CHECK-ARG
+;;; for approval.
+(define (check-arg pred val caller)
+  (if (pred val) val
+      (check-arg pred
+		 (loophole :value (error "Bad argument" val pred caller))
+		 caller)))