doc: sum types

doc/docs.md

@@ -4,14 +4,18 @@
 
 V is a statically typed compiled programming language designed for building maintainable software.
 
-It's similar to Go and is also influenced by Oberon, Rust, Swift.
+It's similar to Go and is also influenced by Oberon, Rust, Swift, Python.
 
 V is a very simple language. Going through this documentation will take you about half an hour,
 and by the end of it you will learn pretty much the entire language.
 
+The language promotes writing simple and clear code with a minimal amount of abstractions.
+
 Despite being simple, it gives a lot of power to the developer. Anything you can do in other languages,
 you can do in V.
 
+
+
 ## Hello World
 
 ```v
@@ -945,6 +949,46 @@ color = .green
 println(color) // "1"  TODO: print "green"?
 ```
 
+## Sum types
+
+```v
+type Expr = BinaryExpr | UnaryExpr | IfExpr 
+
+struct BinaryExpr{ ... }
+struct UnaryExpr{ ... }
+struct IfExpr{ ... }
+
+struct CallExpr {
+	args []Expr
+	...
+}
+
+fn (p mut Parser) expr(precedence int) ast.Expr {
+	match p.tok {
+		.key_if { return IfExpr{} }
+		...
+		else    { return BinaryExpr{} }
+	}
+}
+
+fn gen(expr Expr) {
+	match expr {
+		.key_if { gen_if(it) }
+		...
+	}
+}
+
+fn gen_if(expr IfExpr) {
+	...
+}
+```
+
+To check whether a sum type is a certain type, use `is`:
+
+```v
+println(expr is IfExpr)
+```
+
 ## Option/Result types & error handling
 
 ```v