87 lines
3.6 KiB
Org Mode
87 lines
3.6 KiB
Org Mode
#+TITLE: Software Design for Flexibility
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#+Author: Yann Esposito
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#+Date: [2021-04-03]
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- tags :: [[file:2020-06-03--19-49-30Z--programming.org][programming]]
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- source ::
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My notes on the book.
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Title: Software Design for Flexibility
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Subtitle: How to Avoid Programming Yourself into a Corner
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Authors: Chris Hanson & Gerald Jay Sussman
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* Chapter 1
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Page 19
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In computer science we are taught that the "correctness" of software is
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paramount, and that correctness is to be achieved by establishing formal
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specification of components and systems of componenents and by providing
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proofs that the specifications of a combination of components are met by
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the specification of the components and the pattern by which they are
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combined[^19].
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We assert that this discipline makes systems more brittle.
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In fact to make truly robust systems we must discard such a tight
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discipline.
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[^19]: It is hard, and perhaps impossible, to specify a commplex system. As
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noted on page 7, it is easy to specify that a chess player must play legal
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chess, but how would we specify that it plays well? And unlike chess, whose
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rules do not change, the specifications of mmost system are dynamically
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changing as the conditions of their usage change. How do we specify an
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accounting system in the light of rapidly changing tax codes?
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* Chapter 2
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This chapter introduce a notion of code organisation it is name DSL.
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Domain Specific Language.
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The first part was quite difficult to dig into.
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It uses someting that looks like lost discussion on function arity.
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While the subjet is quite important to me "combinators" them made it
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shallow and not that interesting.
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I was not happy.
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They lost a lot of energy tacklink the tedious problem of function arities.
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Coming from Haskell every function only has a single argument and thus
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makes this issue trivially fixed.
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Here they used a quite terrible approach to anotate the arities on a global
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set hash-map.
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They also use advanced Scheme functions that can provide the min and max
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nunber of argument of a function.
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Let just say, i wasn't really impressed.
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They just declared =comp= and some parallel combinators.
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I think they should have probably left the arities details in the appendice.
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The second part of this chapter is about generating a composable language
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to generate clean regular expressions.
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This one was a lot better.
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They show how the current implementation of regular expression was
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generally terrible and provide a nice composable DSL for it.
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Not something totally new, but it was better than a SQL DSL while similar
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in spirit.
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The third part of this chapter discuss about writing a system for a board
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game rules.
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As a non-native English speaker I haven't understood that /referee/ is the
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English name of the game "les dames".
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So as they also mention a "generic board game" and mention chess, I was
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quite confused by the code and choices I saw.
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So once I understood it.
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It was a nice example about what not to do while designing a system.
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They give two pass of the same code.
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In the first pass, the rules and the executions are mixed together.
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They explain how to fix this shortcoming and made another pass where they
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split the system in three parts.
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A generic "board-game" system, that could be used by most board games.
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An execution system that would take care of applying rules using the board
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game system API.
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And lastly a system to only expres rules that could be used by the previous
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system.
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The interesting part being that you can adapt the rules from referee to
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chess by only changing the rules and not the rest of the code.
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There are still visible problem with this architecture.
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But it was a pretty nice introduction to show how composability is kind of
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preferable than a specialized system.
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