Add procedures for referencing specifier value.

1 files changed, 18 insertions, 2 deletions

diff --git a/module/hnh/util/values.scm b/module/hnh/util/values.scm
index 79f06cff..75db638f 100644
--- a/module/hnh/util/values.scm
+++ b/module/hnh/util/values.scm
@@ -1,7 +1,11 @@
 (define-module (hnh util values)
   :use-module (ice-9 control)
-  :export (abort* on-fst on-snd apply/values)
-  )
+  :use-module (hnh util lens)
+  :export (abort* on-fst on-snd
+                  on-nth
+                  value-ref
+                  value-refx
+                  apply/values))
 
 
 (define-syntax-rule (abort* form)
@@ -21,7 +25,19 @@
      (lambda (prompt fst snd . rest)
        (apply values fst (prompt snd) rest))))
 
+(define-syntax-rule (on-nth idx form)
+  (% form
+     (lambda (prompt . rets)
+       (apply values (modify rets (ref idx) prompt)))))
+
+(define-syntax-rule (value-ref form idx)
+  (call-with-values (lambda () form)
+    (lambda returns (list-ref returns idx))))
+
+(define-syntax-rule (value-refx idx form)
+  (value-ref form idx))
 
+;; TODO replace this with apply/mv from srfi-210
 (define-syntax-rule (apply/values proc form)
   (call-with-values (lambda () form)
     proc))