Add when-with and if-with

This is useful for reading from files.

CHANGELOG.md

@@ -17,6 +17,8 @@ All notable changes to this project will be documented in this file.
   in libuv or embedded in a game.
 - Add `defer`
 - Add `assert`
+- Add `when-with`
+- Add `if-with`
 - Fix thread module issue where sometimes decoding a message failed.
 - Fix segfault regression when macros are called with bad arity.
 

src/boot/boot.janet

@@ -304,6 +304,22 @@
      (defer (,(or dtor :close) ,binding)
        ,;body)))
 
+(defmacro when-with
+  "Similar to with, but if binding is false or nil, returns
+  nil without evaluating the body. Otherwise, the same as with."
+  [[binding ctor dtor] & body]
+  ~(if-let [,binding ,ctor]
+     (defer (,(or dtor :close) ,binding)
+       ,;body)))
+
+(defmacro if-with
+  "Similar to with, but if binding is false or nil, returns
+  nil without evaluating the body. Otherwise, the same as with."
+  [[binding ctor dtor] truthy &opt falsey ]
+  ~(if-let [,binding ,ctor]
+     (defer (,(or dtor :close) ,binding) ,truthy)
+     ,falsey))
+
 (defn- for-template
   [binding start stop step comparison delta body]
   (with-syms [i s]

test/suite7.janet

@@ -286,4 +286,17 @@
   (file/seek f :set 0)
   (assert (= (string (file/read f :all)) "foo\n") "temp files work"))
 
+(var counter 0)
+(when-with [x nil |$]
+           (++ counter))
+(when-with [x 10 |$]
+           (+= counter 10))
+
+(assert (= 10 counter) "when-with 1")
+
+(if-with [x nil |$] (++ counter) (+= counter 10))
+(if-with [x true |$] (+= counter 20) (+= counter 30))
+
+(assert (= 40 counter) "if-with 1")
+
 (end-suite)