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ECS140A-L2

3 min read

📗 -> 09/30/25: ECS140A-L2

Lecture Slide Link
Go slides

🎤 Vocab

❗ Unit and Larger Context

Small summary

✒️ -> Scratch Notes

Revised Grammar

Establishing a “precedence cascade” to reduce ambiguity in our example (is it 7 or 9?).

  • Now, it is always 7.
    Can also introduce a new rule to enforce ordering for associativity (solve from left)

Extended BNF (EBNF)

Adding short hands to simplify productions

  • repetition, optional, grouping
Repetition:
# Before
number ::= <digit> | <number> <digit>

# After
<number> ::= >digit> {<digit>}
Optional
# Before
<if-stmt> ::= if <cond> then <stmt>  
			| if <cond> then <stmt> else <stmt>  
# After
<if-stmt> ::= if <cond> then <stmt> [else <stmt>]
Grouping
# Before
<expr> ::= <expr> + <expr> | <expr> - <expr> | <num>
# After
<expr> ::= <expr> (+ | -) <expr> | <num>
Conversions BNF -> EBNF

Recursions

<A> ::= <A> a | <B> 
	=> <A> ::= <B> { a }

Common string to factor out

<A> ::= a <B> | a 
	=> <A> ::= a [<B>]  
	
<A> ::= a <B> | a <C> 
	=> <A> ::= a (<B> | <C>)
Conversions EBNF -> BNF

Options

<A> ::= a [<B>] <C> 
	=> <A> ::= a <C> | a <B> <C>

Repetition: { }

<A> ::= a { <B1> <B2> <Bn> } <C>
	=> <A> ::= a <B> <C>
		<B> ::= <B> <B1> <B2> <Bn> | empty

Grouping

<A> ::= a (<B> | <C>) <D>
	=> <A> ::= a <B> <D> | a <C> <D>

Go

Declarations and scope

Declaration - Associates a name with a program entity (e.x. funcs or vars)
Scope - Part of the code where a use of the declared name refers to that declaration

Syntax

Every go program must belong to a package? boilerplate needed: package main

Importing:
import { "fmt" } - “format”, used for logging fmt.Println("hello")

Declarations

Full Declaration: var name {type | = expression}

  • Can be used for global or local
  • either type or value, type can be inferred

Short declaration: name := expression

  • Only used for local vars

Can include declarations in if statements.

  • They are only able to be referenced in their if/else blocks
if x := f(); x == 0 {
	fmt.Println(x)
}

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