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# Compilers and Interpreters

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CS 135 // 2021-04-06

## Administrivia
- All labs have been graded and returned on codePost
- Please let me know if you have any questions about the grading

# Compilers and Interpreters

## Discussion Questions
1. What is a **compiler**?
2. What is an **interpreter**?
3. What are the differences between them?

## Responses
- A **compiler** is:
    + Takes code from one language and translates to another
    + Only translates (and optimizes)
- An **interpreter** is:
    + Reads and executes a program immediately

## Compiled VS Interpreted
- Which of these languages are compiled and which are interpreted?
    + Python
    + Java
    + C
    + Haskell

## Components
Every compiler/interpreter must perform:

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- 
 **Lexical Analysis** (aka "Lexing", or "Tokenizing")
    + 
 Turns source code into a sequence of **tokens**
      ```java
      boolean result = "123".equals(123);
      ```

## Components
Every compiler/interpreter must perform:

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- **Syntax Analysis** (aka "Parsing")
    + 
 Constructs a **parse tree** from a sequence of tokens according to the grammar of the language
- 
 **Semantic Analysis**
    + 
 Type checking and other compile-time verification

## Outline
- Over the next five weeks, we are going to write an **interpreter** for a new programming language
- 
 We will focus on how features of many programming languages are implemented
- 
 Very little time will be given to lexing/parsing
    + 
 General-purpose parsing libraries and tools exist to perform this

# Features
## of Programming Languages

## Classifying Languages
- We just saw 16 different presentations on programming languages and their features
- 
 What are the common features and categories of programming languages?
- 
 Think about what all the "buzzwords" were in these presentations that differentiate them
- 
 In your small groups, make a list of these buzzwords/categories along with brief definitions of what they mean
- 
 We will discuss them as a large group after

## Responses
- Strong typing / weak typing
- Static typing / dynamic typing
    + Static means that your types are checked at compile-time
    + Dynamic means that your types are checked at runtime
- Object-oriented, functional, procedural, imperative
    + OO is a very specific style of programming where objects contain both data and methods for manipulating that data

## Responses
- Open source / closed source
- High-level / low-level / machine-level
- Pure / impure (side-effects)

## Languages as a Collection of Features
- These days, languages are big "blobs" of features
- 
 When learning a new language, it is helpful to learn its basic features
- 
 Every language has a (relatively) simple "core" language that it builds from

## Desugaring
- We will develop the core of a programming language interpreter, one feature at a time
- 
 We will also be adding "syntactic sugar" to the language and a desugaring process to reduce more complicated features to the core features