From 1773b32d94a58b0a6fbf7f9058886dbba03b73a2 Mon Sep 17 00:00:00 2001
From: Johannes Loher <johannes.loher@maibornwolff.de>
Date: Thu, 7 Jun 2018 23:08:10 +0200
Subject: [PATCH] add section about conditional compilation

---
 hands-on_dlang.md | 102 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 98 insertions(+), 4 deletions(-)

diff --git a/hands-on_dlang.md b/hands-on_dlang.md
index baeb8d9..e798c72 100644
--- a/hands-on_dlang.md
+++ b/hands-on_dlang.md
@@ -545,7 +545,7 @@ flag `--boundcheck=off`.
 #### Static arrays
 
 Static arrays are stored on he stack or in static memory, depending on where
-they are defined. They have a fixed, compile-time known length. The length is
+they are defined. They have a fixed, compile time known length. The length is
 part of the type:
 
 ```D
@@ -1357,8 +1357,8 @@ the `-cov` flag. When using DUB, just add the flag `--coverage`.
 
 ### String mixins
 
-The mixin expression takes an arbitrary string and compiles it and generates instructions accordingly. This is a compile-time feature, so ths string needs
-to be known at compile-time.
+The mixin expression takes an arbitrary string and compiles it and generates instructions accordingly. This is a compile time feature, so ths string needs
+to be known at compile time.
 
 ```D
 mixin("int b = 5"); // compiles just fine
@@ -1369,7 +1369,7 @@ mixin(compileTimeKnownString); // so does this
 assert(c == 5);
 
 immutable runTimeString = q{int c =5;};
-mixin(runTimeString); // compile error, runTimeString is not known at compile-time
+mixin(runTimeString); // compile error, runTimeString is not known at compile time
 ```
 
 ### Compile Time Function Evaluation (CTFE)
@@ -1412,4 +1412,98 @@ void main() {
     writeln("The sqrt of compile time 4 = ",
         cn);
 }
+```
+
+### Conditional compilation
+
+D has several methods of conditional compilation which can be used for code
+generation. D’s traits and the module `std.traits` contain many usefull tools
+which can be used with the following constructs.
+
+#### `static if` & `is`
+
+`static if` conditionally compiles a code block based on a condition that can be
+evaluated at compile time:
+
+```D
+foo(int x)() { // x is a template parameter and thus known at compile time
+    static if(x == 0)
+        writeln("x is 0");
+    else
+        writeln()
+}
+```
+
+The `is` expression is used to check for valid types and to compare types:
+
+```D
+foo(T)() {
+    static if(is(T)) {
+        writeln("T is a valid type");
+    }
+    static if(is(T == int)) {
+        writeln("T is int");
+    }
+    static if(is(T : int)) {
+        writeln("T implicitly converts to int");
+    }
+}
+```
+
+#### `static foreach`
+
+`static foreach` allows you to iterate at compile time in order to generate
+code. It is best explained by an example: The code
+
+```D
+import std.stdio;
+
+void main() {
+    static foreach(element; ["foo", "bar", "baz"]) {
+        writeln(element);
+    } 
+}
+```
+
+effectively lowers to
+
+```D
+import std.stdio;
+
+void main() {
+    writeln("foo");
+    writeln("bar");
+    writeln("baz");
+}
+```
+
+which means there is no loop at runtime.
+
+#### Template constraints
+
+A template may be defined with any number of constraints that enforce some
+properties of the template parameters:
+
+```D
+void foo(T)(T value)
+    if (is(T : long)) { // foo!T only valid if T implicitly converts to long
+}
+
+foo(42); // compiles, 42 is of type int which implicitly converts to long
+foo("someString"); // compile error, strings do not implicitly convert to long
+```
+
+```D
+void bar(T)(T value)
+    if (__traits(compiles, {T t = T.init; t.fly();})) { // bar!T only valid if we can initialize T and T has a method called fly which takes no parameters
+}
+
+struct MyStruct {
+    void fly() {}
+}
+
+MyStruct myStruct;
+
+bar(myStruct); // compiles
+bar(42); // compile error
 ```
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