2016年的最后几小时, 随便写写关于Scala和OCaml的一些入门体验好了.

今年对FP语言特别感兴趣, 上了两门Scala的公开课(here and here)和一门OCaml的公开课(here), 在博客中写了一系列的笔记, 课后作业也都认真做完了. 斗胆说这两门语言都算入门了吧... 这里就随便写一下使用这两门语言的感受, 想到哪里写到哪里...

FP语言和之前接触的语言确实不大一样, 比如之前我都有种错觉, 学什么语言只要知道循环/条件/基本类型运算怎么写, 就差不多可以上手了...... 然后遇到了FP, 发现循环语句其实是不必要的... 记得看到过一篇文章, 类比学FP就好像开了很多种车的老司机突然开始学开宇宙飞船, 肯定各种WTF不适应了~

以前谈到FP我只能联想到一些Python里的FP特性: lambda表达式, 高阶函数之类的, 顶多还想到个闭包... 不过Python里面的FP特性和Scala/OCaml里的比起来还是差了不少: i.e. 现在非常希望Python里可以支持pattern matching...

Scala

Scala算是比较亲民的FP语言了(和Java有点像...), 也是我最早接触的FP语言. EPFL的那两门公开课质量很棒, 毕竟是Scala的作者亲自来上的...

• immutable types: 习惯了就好, 就像java里所有东西都是final的, 要修改什么东西的时候改成新建一个, immutable数据的优点就是并行方便啊...
• 一切皆为表达式, specifically, if ...

this week: programming-in-the-large using the module system of OCaml.

1. STRUCTURING SOFTWARE WITH MODULES

in large project: mangage high number of definitions → abstractions built on top of other abstractions.

• layers of abstractions: hide information
• divide program into components
• identifiers organised to avoid naming conflicts

module as namespace

dot-notation: access module ...

1. IMPERATIVE FEATURES IN OCAML

functional language:

• immutable data structure
• identifiers instead of variables
• pure functions

but imperative features are useful:

• exceptions to alter control flow
• ops to consume input and output
• mutable data structures
• for and while loop for iterations

2. GETTING AND HANDLING YOUR EXCEPTIONS

• exceptions in ocaml ...

1. FUNCTIONAL EXPRESSIONS

syntax for functional expr: function some_identifier -> some_expr
the type of the functional expr is t1 -> t2 where t1 is the type of some_identifier, t2 is type of some_expr

ex.

function x -> x + 1;;  
(function x -> 2*x) 5;; (*annonymous function*)   

the previous way of defining function:

let ...

Last week, we only defined flat data structures which are nice to aggregate values but quite limited when you try to structure values.

This week: algebraic datatypes.

1. TAGGED VALUES

⇒ change the return type to a type query_result, which can be either of these:

• an error
• a new database (in ...

this week: structure code with types: tuples, records, arrays.

1. USER-DEFINED TYPES

primary use of types: document your code

• use type type_identifier = some_type to define a new type (type_identifier is synonym/abbrevation of some_type)
• type_identifier must start with lowercase letter
• already known types: int, bool, string, char ...
• use : to add ...

1. BASIC DATA TYPES: int, bool

Rich type system and polymorphism in ocaml. Types are infered not declared.

Basic types: int, bool, float, string, char, ...

int

value: \(-2^{62}\) ~ \(2^{62}-1\) on 64-bit machines.
ops: +, -, *, /, mod (reminder: / is integer division)

# 3+2*4-1;; 
- : int = 10 
# 5/2;; 
- : int ...

Intro

ML lang family

statically strongly typed languages

• fisrt-class functions
• type inference
• pattern matching

highlights of ocaml

• safty: static typing, pattern matching
• efficiency: high performance

• expressiveness: functional+type inference+polymorphism

• package manager: opam

• debugger
• profiler
• REPL: "toplevel"
• bytecode compiler: ccamlc
• native compiler: ocamlopt

programming environment fully online: https://try.ocamlpro ...