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Introduction
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About this course
=================

* Advanced subjects -- for 3rd or 4th year students.
* Aim: Theoretical understanding of programming languages.
* Prerequisite: knowledge about two or more programming languages.

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Evaluation
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Allotment of points
~~~~~~~~~~~~~~~~~~~~~~

* Weelky assignments: 20points
* Mid-term paper and presentation in class: 40points
* End-of-term exam: 40points

Distribution in the past years
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

* year 2021 --- S:6, A:1, B:2, C:2, D:2
* year 2022 --- S:13, A:16, B:5, C:3, D:13
  
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A Fundamental Question
======================

Why do we have so many programming languages?

* `List of programming languages - Wikipedia <https://en.wikipedia.org/wiki/List_of_programming_languages>`_ 
* New languages such as Go, Swift, and Rust has been developed recently.
* These languages are different from each other, but how different?

.. _definition_of_language:

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Definition of a language
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When you want to define a new language, it is important that the definition is simple, rigorous and understandable.

* Scheme gives a mathematically precise `definition <http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-10.html#%_chap_7>`_ which is hard to understand.
* BASIC and C had no rigid definitions at first.  As a result, we had several slightly different language processors. 

We need two things to define a language.

* :index:`syntax` --- how you write a program.
* :index:`semantics` --- what will happen when executiing a program

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Differences in syntax
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Character
~~~~~~~~~

* Unique symbols (cf. `APL <http://wiki.c2.com/?AplLanguage>`_ )
* Japanese characters (cf. `Mind <http://www.scripts-lab.co.jp/mind/whatsmind.html>`_ )
* Graphics instead of characters (cf. `ToonTalk <https://toontalk.github.io/ToonTalk/>`_ , `Viscuit <http://www.viscuit.com/>`_ )

Grammar
~~~~~~~

There are languages called `Esoteric progarmming language <https://en.wikipedia.org/wiki/Esoteric_programming_language>`_.

* `Whitespace <https://github.com/Romejanic/Whitespace/blob/master/helloworld.ws>`_
* `Befunge <http://en.wikipedia.org/wiki/Befunge>`_

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Differences in semantics
========================

Philosophically speaking, programming paradigm means "What is computation?".

Imperative (Procedural)
  Computation is a sequence of changing state.

Functional
  Computation is a reduction of expression.

Logical
  Computation is a proof of proposition.

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Criteria for good programming languages
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* Readablility and writability are not compatible in general.
* Efficiency

  * fast execution
  * fast compilation
  * fast development process

* Reliability

  * Strong typing, forcing execption handling, etc.  (However, some people don't like them)

* Other points

  * low learning cost
  * low development cost of language processors
  * abundant libraries

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Reasons to study the theory of programming language
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* You can select a language suitable for a task in your hand. --- There is a saying 'If all you have is a hammer, everything looks like a nail'.
* You can learn a new language easily. --- You must continue to learn since progress of IT is very fast.
* It is an important base of the computer science.