added scheme/nest

svn: r9085

collects/scheme/nest.ss

@@ -0,0 +1,28 @@
+#lang scheme/base
+
+(provide nest)
+
+;; A form that makes it easy to combine many "right-drifting" nested
+;; expressions into a single one, sort of like a generalized version of the `*'
+;; from `let*' and similar forms.
+(define-syntax nest
+  (syntax-rules ()
+    [(nest () body0 body ...)
+     (let () body0 body ...)]
+    ;; this allows putting definitions in the body
+    [(nest ([form forms ...]) body0 body ...)
+     (form forms ... (let () body0 body ...))]
+    [(nest ([form forms ...] . more) body0 body ...)
+     (form forms ... (nest more body0 body ...))]))
+
+;; using this instead will allow splicing body expressions in the last form
+;; whatever it happens to be, which can be (ab)used in strange ways
+#;
+(define-syntax nest
+  (syntax-rules ()
+    [(nest () body ...)
+     (begin body ...)]
+    [(nest ([form forms ...]) body ...)
+     (form forms ... body ...)]
+    [(nest ([form forms ...] . more) body ...)
+     (form forms ... (nest more body ...))]))