From 77923a4d12717902137f8cd29a512796eff1a3af Mon Sep 17 00:00:00 2001
From: Melody Horn <melody@boringcactus.com>
Date: Fri, 2 Apr 2021 19:28:31 -0600
Subject: implement `if`

---
 src/makefile/functions.rs | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/src/makefile/functions.rs b/src/makefile/functions.rs
index a4d206f..bc244cb 100644
--- a/src/makefile/functions.rs
+++ b/src/makefile/functions.rs
@@ -36,6 +36,11 @@ pub fn expand_call(name: &str, args: &[TokenString], macros: &MacroSet) -> Resul
             file_name::wildcard(macros, &args[0])
         }
 
+        "if" => {
+            assert!(args.len() == 2 || args.len() == 3);
+            conditional::r#if(macros, &args[0], &args[1], args.get(2))
+        }
+
         // foreach
         "foreach" => {
             assert_eq!(args.len(), 3);
@@ -183,6 +188,32 @@ mod file_name {
     }
 }
 
+// Functions for Conditionals
+mod conditional {
+    use super::*;
+
+    pub fn r#if(
+        macros: &MacroSet,
+        condition: &TokenString,
+        if_true: &TokenString,
+        if_false: Option<&TokenString>,
+    ) -> Result<String> {
+        let mut condition = condition.clone();
+        condition.trim_start();
+        condition.trim_end();
+        let condition = macros.expand(&condition)?;
+        if condition.is_empty() {
+            if let Some(if_false) = if_false {
+                macros.expand(if_false)
+            } else {
+                Ok(String::new())
+            }
+        } else {
+            macros.expand(if_true)
+        }
+    }
+}
+
 // foreach
 mod foreach {
     use super::*;
-- 
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