This project file is needed in the root directory of every source tree that you want to compile with Dune. In general, you’ll have a dune file in every subdirectory of the source tree but only one dune-project file at the root.
dune 文件用来制定每个 sub-directory 如何编译；而dune-project 文件用来指定整个项目的编译配置
let increment x = x + 1
Ocaml 中函数定义的语法和变量定义的语法基本一致，这让 “函数也是一种数据” 变得更加显然。
There are five essential components to learning a language: syntax, semantics, idioms, libraries, and tools.
掌握语言的五个要点：语法规则、语义表示、习语定式、库、工具
Parametric polymorphism: Functions and data structures can be parameterized over types. This is crucial for being able to re-use code.
参数化多态，也就是，范型