From 962cd7e5f5e5047385c75557a6a7bdd3709116e1 Mon Sep 17 00:00:00 2001
From: Calvin Rose <calsrose@gmail.com>
Date: Wed, 15 Jan 2020 22:49:41 -0600
Subject: [PATCH] Add when-with and if-with

This is useful for reading from files.
---
 CHANGELOG.md        |  2 ++
 src/boot/boot.janet | 16 ++++++++++++++++
 test/suite7.janet   | 13 +++++++++++++
 3 files changed, 31 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3a175acc..8b3e570a 100644
--- a/CHANGELOG.md
+++ b/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.
 
diff --git a/src/boot/boot.janet b/src/boot/boot.janet
index 10ff904b..6d8f5625 100644
--- a/src/boot/boot.janet
+++ b/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]
diff --git a/test/suite7.janet b/test/suite7.janet
index 520210aa..cdffc3c9 100644
--- a/test/suite7.janet
+++ b/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)