# HG changeset patch
# User nipkow
# Date 1180157163 -7200
# Node ID f5de7da165bb1e2f147358691efe94f35624ea4c
# Parent  669d7391df1a1cf1a08d330b64089a3c416b9479
What it says

diff -r 669d7391df1a -r f5de7da165bb src/HOL/Library/List_Comprehension.thy
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/HOL/Library/List_Comprehension.thy	Sat May 26 07:26:03 2007 +0200
@@ -0,0 +1,50 @@
+(*  Title:      HOL/Library/List_Comprehension.thy
+    ID:         $Id$
+    Author:     Tobias Nipkow
+*)
+
+header {* List Comprehension *}
+
+theory List_Comprehension
+imports Main
+begin
+
+text{* At the moment this theory provides only the input syntax for
+list comprehension: @{text"[x. x \<leftarrow> xs, \<dots>]"} rather than
+\verb![x| x <- xs, ...]! as in Haskell.
+
+The print translation from internal form to list comprehensions would
+be nice. Unfortunately one cannot just turn the translations around
+because in the final equality @{text p} occurs twice on the
+right-hand side. Due to this restriction, the translation must be hand-coded.
+
+A more substantial extension would be proper theorem proving
+support. For example, it would be nice if
+@{text"set[f x y. x \<leftarrow> xs, y \<leftarrow> ys, P x y]"}
+produced something like
+@{term"{z. EX x: set xs. EX y:set ys. P x y \<and> z = f x y}"}.  *}
+
+nonterminals lc_gentest
+
+syntax
+"_listcompr" :: "'a \<Rightarrow> idt \<Rightarrow> 'b list \<Rightarrow> lc_gentest \<Rightarrow> 'a list" ("[_ . _ \<leftarrow> __")
+"_lc_end" :: "lc_gentest" ("]")
+"_lc_gen" :: "idt \<Rightarrow> 'a list \<Rightarrow> lc_gentest \<Rightarrow> lc_gentest" (",_ \<leftarrow> __")
+"_lc_test" :: "bool \<Rightarrow> lc_gentest \<Rightarrow> lc_gentest" (",__")
+
+
+translations
+ "[e. p\<leftarrow>xs]" => "map (%p. e) xs"
+ "_listcompr e p xs (_lc_gen q ys GT)" =>
+  "concat (map (%p. _listcompr e q ys GT) xs)"
+ "_listcompr e p xs (_lc_test P GT)" => "_listcompr e p (filter (%p. P) xs) GT"
+
+(*
+term "[(x,y). x \<leftarrow> xs, x<y]"
+term "[(x,y). x \<leftarrow> xs, x<y, z\<leftarrow>zs]"
+term "[(x,y). x \<leftarrow> xs, y \<leftarrow> ys, x<y]"
+term "[(x,y,z). x \<leftarrow> xs, y \<leftarrow> ys, z\<leftarrow> zs]"
+term "[x. x \<leftarrow> xs, x < a, x > b]"
+*)
+
+end