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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 …

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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 …

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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 …

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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 …

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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 …

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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 …

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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 …

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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 …