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《Learn You a Haskell for Great Good!》 study note

Thu, 18 May 2017 23:35:08 +0000

Introduction

Haskell is a purely functional programming language.
"In purely functional programming you don't tell the computer what to do as such but rather you tell it what stuff is. … You express that in the form of functions. … "
A Haskell function cannot have side effects other than mapping its input to an output as a mathematical function.
Haskell is lazy in that it does not compute anything until it have to in order to return a value.
Haskell is statically typed.
- Type inference allows deducing the appropriate type automatically by inference on compatibility between the inputs and outputs.

Starting out

Ready, set, go!

ghci: The Glorious Glasgow Haskell Compilation System. Environment to interactively compile and run Haskell programs. Exit GHCi using :quit. GHCi documentation.
Not-equal operator: /=.
Compile-time error on incompatible types: 123 + "foobar"
Compile a program: ghci> l:myprogram.hs.
' is allowed in function name, e.g. foo'Bar is a valid function name.
Function names cannot begin with a lower-case letter.

An intro to lists

In Haskell, a strings is a lists of characters.
List and tuple syntax difference: [1, 2], (1, "foobar"). A tuple can contain elements of different types, while a list cannot. This bears some similarity with Python's lists and tuples. A difference is that in Python, lists are mutable but tuples are not.
Concatenate operation: [1, 2] ++ [3, 4], "foo" ++ "bar", ['a', 'b'] ++ ['c', 'd'].

[https://en.wikipedia.org/wiki/Cons cons] operation ::

  Prelude> 'f' : "bar"
  "fbar"
  Prelude> 1: [3, 4]
  [1,3,4]
  Prelude>

If "XXX...XXX" is a very long string, "XXX...XXX" ++ "foobar" will be slow.
[1, 2, 3] equals 1 : 2 : 3 : [].
Take parts of a list: [1, 3, 5, 7] !! 2 returns 5. Haskell lists uses zero-based numbering. take 2 [1, 3, 5, 7] gives the third element 5. drop 2 [1, 3, 5, 7] gives [1, 3, 7]. Compare this to Mathematica {1,3,5,7}<span class="createlink">3</span>, Take[{1,3,5,7}, {3}], Drop[{1,3,5,7}, {3}].
Test element membership in a list: elem 3 [2, 4, 6] and 2 \elem` [1, 3, 5, 7]`.
[[1, 2], ["foo", "bar"]] is invalid as the types of the elements of the two sub-lists are not same.
Lists compare in lexicographical order. E.g. [1, 3, 4] < [1, 4, 2].
head [1, 2, 3], tail [1, 2, 3], init [1, 2, 3], last [1, 2, 3]. Compare with Mathematica / Wolfram Langauge namings: First[{1,2,3}], Rest[{1,2,3}], Most[{1,2,3}], Last[{1,2,3}]
null [1, 2, 3], null []

Other side notes

One of the key features and advantanges of Haskell is its purity: all side-effects are encapsulated in a monad.
[https://en.wikipedia.org/wiki/Algebraic_data type Algebraic data type]:

data List a = Nil | Cons a (List a)

data Tree = Empty | Leaf Int | Node Tree Tree

Tree is a sum data type a.k.a. tagged union.

Tree is a recursive data type.

Pattern matching in Haskell:

  depth :: Tree -> Int
  depth Empty = 0
  depth (Leaf n) = 1
  depth (Node l r) = 1 + max (depth l) (depth r)

References

https://ghcformacosx.github.io/
Search a Haskell function by desired type signature, e.g. https://www.haskell.org/hoogle/?hoogle=String+-%3E+Int (And the desired function for turning an object to a String is possibly read :: Read a => String -> a).

Manufacturing Consent

Sat, 08 Apr 2017 22:21:45 +0000

I learned about the book 《Manufacturing Consent: The Political Economy of the Mass Media》 (1988), by Edward S. Herman and Noam Chomsky (website) from Aaron Swartz's blog post 〈The Book That Changed My Life〉. His dramatic description of how he felt after reading the book and watching the documentary film made me extremely interested in them too. I've watched interview to Chomsky in the past but have never read his books.

download the torrent file for the documentary film video "Manufacturing Consent: Noam Chomsky and the Media"
a list of some books by Noam Chomsky on goodreads.com
another famous book by Chomsky 《Understanding Power - The Indispensable Chomsky》(Amazon)
- the companion website: http://understandingpower.com/

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