nzkrdev study group 20200731

what-if.md

What If?

@microamp

Preface: Syntax vs Semantics

• Syntax:

  the grammatical rules and structural patterns governing the ordered use of appropriate words and symbols for issuing commands, writing code, etc.

• Semantics:

  describes the processes a computer follows when executing a program in that specific language

Let's talk about if

• Every (high-level) programming language has if

• Example (in Ruby):

  if raining then
    puts("home")
  else
    puts("school")
  end
  

if: statement vs expression

• An expression always returns a value

• Examples:

  • Python: with if

    "home" if raining else "school"
    

  • JavaScript: without if

    raining ? "home" : "school"
    

  • Racket: Lisp dialect, (impure) functional

    (if raining "home" "school")
    

• Syntax rules vary, but they're semantically the same (kind of…)

Let's talk about unless

• unless = the opposite of if

  • "I'm not going to school if it's raining"
  • "I'm going to school unless it's raining"

• Examples (in Ruby):

  if raining then "home" else "school" end
  
  unless raining then "school" else "home" end
  

Every programmer has…

this superpower

which is…

• the ability to define their own function
• define, function, def, defn, func, fn, etc.

So, I really, really like unless…

• but my favourite language (not Ruby) doesn't have it :(

• Solution: Let's define it as a function! :)

• Checklist [2/2]

  • if
  • define (for defining a function)

• Both are already provided by your language!

unless as a function

Recall if in Racket

(if e1 e2 e3)

compared to if in Ruby

if e1 then e2 else e3 end

unless as a function (cont)

• define syntax in Racket

  (define (fn-name arg1 arg2 arg3 ...)
    e)
  

• unless: The implementation

  (define (unless e1 e2 e3)
    (if e1 e3 e2))
  

  (Note: e2 and e3 are reversed)

unless as a function (cont)

Let's run some tests…

(let ([raining #t])
  (unless raining
    "school"
    "home")) ;; "home"

(let ([raining #f])
  (unless raining
    "school"
    "home")) ;; "school"

Hooray!

So… that's it?

• Q: Is our unless function semantically same as Ruby's unless?

Let's Compare

> unless true then 6 / 2 else 1 + 2 + 3 + 4 end
=> 10

> unless false then 6 / 2 else 1 + 2 + 3 + 4 end
=> 3

Let's Compare (cont)

> unless true then 6 / 0 else 1 + 2 + 3 + 4 end

?

Let's Compare (cont)

> unless true then 6 / 0 else 1 + 2 + 3 + 4 end
=> 10

Let's Compare (cont)

(unless #t
  (/ 6 0)
  (+ 1 2 3 4))

; /: division by zero

which is not how Ruby's unless works :(

The Problem

• unless cannot be defined as a function because function arguments are evaluated before they are added to the body of the function
• Unnecessary computation (what if e2 requires a very expensive computation)

What If?

• You, the programmer
  • can create new syntax rules at runtime (new keywords, such as unless)
  • i.e., adding extentions to the host language

Macros

A macro is a syntactic form with an associated transformer that expands the original form into existing forms. To put it another way, a macro is an extension to the Racket compiler. - https://docs.racket-lang.org/guide/macros.html

The primary use of macros in programming languages is to extend or adapt the language to allow a more concise or readable solution for a particular problem domain.

#demotime

unless as a macro

Syntax:

(define-syntax ...
  (syntax-rules ()
    [(...)
     (...)]))

unless as a macro

(define-syntax unless
  (syntax-rules ()
    [(unless e1 e2 e3)
     (if e1 e3 e2)]))

(unless #t (/ 6 0) (+ 1 2 3 4)) ;; 10

Wrap-Up

• Macros are not functions

• Macros are used for defining language syntax extensions to the base language (runtime code generation, aka macro expansion)

  • Great for writing DSLs (domain-specific languages)

• "With great power comes great responsibility"

  • "The First Rule of Macro Club. You do not write macros."