In the lazy branch, empty sequences don't always evaluate as
false in a boolean context. Tracking down places you've
made this assumption can be hard. Attached is a patch that
helps by providing a flag, assert-if-lazy-seq.
When this flag is on, 'if' is replaced with a new (slower)
version that throws an exception if you're using a lazy-seq
as the test expression. This is almost always an error in
the lazy branch, and usually just requires wrapping the
lazy-seq in a call to 'seq', as has been discussed
elsewhere.
To turn on the flag you need to rebuild clojure with an
extra option, like this:
ant -Dclojure.assert-if-lazy-seq=please
Any non-empty string will do for the value.
You will need to set the value at runtime as well. There
may be a way to avoid this, but I haven't thought of how.
So for now you'll want at the top of your code:
(reset! *assert-if-lazy-seq* true)
Now anytime you try to use nil punning, you should get an
exception.
user=> (when (filter neg? [1 2]) :all-pos)
java.lang.Exception: LazySeq used in 'if' (NO_SOURCE_FILE:0)
user=> (not (concat))
java.lang.Exception: LazySeq used in 'if' (NO_SOURCE_FILE:0)
--Chouser
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commit b0cec18fb925510502352d8fdcc0a4b93b28bd72
Author: Chouser <chou...@n01se.net>
Date: Fri Feb 13 13:02:30 2009 -0500
Add assert-if-lazy-seq
diff --git a/branches/lazy/build.xml b/branches/lazy/build.xml
index 04583ba..bc5fcc7 100644
--- a/branches/lazy/build.xml
+++ b/branches/lazy/build.xml
@@ -11,6 +11,7 @@
<property name="build" location="classes"/>
<property name="clojure_jar" location="clojure.jar"/>
<property name="slim_jar" location="clojure-slim.jar"/>
+ <property name="clojure.assert-if-lazy-seq" value=""/>
<target name="init" depends="clean">
<tstamp/>
@@ -28,6 +29,7 @@
<java classname="clojure.lang.Compile"
classpath="${build}:${cljsrc}">
<sysproperty key="clojure.compile.path" value="${build}"/>
+ <sysproperty key="clojure.assert-if-lazy-seq" value="${clojure.assert-if-lazy-seq}"/>
<arg value="clojure.core"/>
<arg value="clojure.main"/>
<arg value="clojure.set"/>
diff --git a/branches/lazy/src/clj/clojure/core.clj b/branches/lazy/src/clj/clojure/core.clj
index eb31005..8f2ec15 100644
--- a/branches/lazy/src/clj/clojure/core.clj
+++ b/branches/lazy/src/clj/clojure/core.clj
@@ -36,6 +36,10 @@
fn (fn* fn [& decl] (cons 'fn* decl)))
(def
+ #^{:macro true}
+ if (fn* if [& decl] (cons 'if* decl)))
+
+(def
#^{:arglists '([coll])
:doc "Returns the first item in the collection. Calls seq on its
argument. If coll is nil, returns nil."}
@@ -291,6 +295,19 @@
(. (var defmacro) (setMacro))
+(def *assert-if-lazy-seq*
+ (let [x (System/getProperty "clojure.assert-if-lazy-seq")]
+ (new clojure.lang.Atom (if x (if (.isEmpty x) false true)))))
+
+(defmacro if [tst & etc]
+ (if* (.get *assert-if-lazy-seq*)
+ (let [tstsym 'no-gensym-for-me]
+ (list 'let [tstsym tst]
+ (list 'if* (list 'instance? clojure.lang.LazySeq tstsym)
+ (list 'throw (list 'new Exception "LazySeq used in 'if'"))
+ (cons 'if* (cons tstsym etc)))))
+ (cons 'if* (cons tst etc))))
+
(defmacro when
"Evaluates test. If logical true, evaluates body in an implicit do."
[test & body]
@@ -1642,7 +1659,7 @@
([coll]
(sort compare coll))
([#^java.util.Comparator comp coll]
- (when (and coll (not (zero? (count coll))))
+ (when (seq coll)
(let [a (to-array coll)]
(. java.util.Arrays (sort a comp))
(seq a)))))
diff --git a/branches/lazy/src/clj/clojure/core_proxy.clj b/branches/lazy/src/clj/clojure/core_proxy.clj
index 8517e23..edbc78f 100644
--- a/branches/lazy/src/clj/clojure/core_proxy.clj
+++ b/branches/lazy/src/clj/clojure/core_proxy.clj
@@ -155,7 +155,7 @@
meths (concat
(seq (. c (getDeclaredMethods)))
(seq (. c (getMethods))))]
- (if meths
+ (if (seq meths)
(let [#^java.lang.reflect.Method meth (first meths)
mods (. meth (getModifiers))
mk (method-sig meth)]
diff --git a/branches/lazy/src/jvm/clojure/lang/Compiler.java b/branches/lazy/src/jvm/clojure/lang/Compiler.java
index 664045f..b29876f 100644
--- a/branches/lazy/src/jvm/clojure/lang/Compiler.java
+++ b/branches/lazy/src/jvm/clojure/lang/Compiler.java
@@ -40,7 +40,7 @@ public class Compiler implements Opcodes{
static final Symbol DEF = Symbol.create("def");
static final Symbol LOOP = Symbol.create("loop*");
static final Symbol RECUR = Symbol.create("recur");
-static final Symbol IF = Symbol.create("if");
+static final Symbol IF = Symbol.create("if*");
static final Symbol LET = Symbol.create("let*");
static final Symbol DO = Symbol.create("do");
static final Symbol FN = Symbol.create("fn*");
