Lec32-Macros

HHZZ included in UCB-CS61A

2024-10-23 178 words One minute

Contents

Macros

quote or unquote

在于怎么eval 🤔

, 后面的 (some_expression) 不会被quote

Macros in Scheme

define new special form in scheme…

define-macro 定义一个新的宏，接收一个 <expression> 作为参数，返回一个新的<expression> 并且eval之, 先不对参数eval

注意和define的区别，define在意的是传进来的 value

another example, to show info of your expression using macro

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(define-macro (check expr)
    (list 'if expr ''pass 
        (list 'quote (list 'failed: expr))))

For Macro in Scheme

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(define (map fn vals)
    (if (null? vals)
        ()
        (cons (fn (car vals))
              (map fn (cdr vals)))))

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> (map (lambda (x) (* x x)) '(2 3 4 5)) ; without using macro case
(4 9 16 25)

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(define-macro (for sym vals expr)
    (list 'map lambda (sym) (expr) (vals)) ;? wrong!
    (list 'map (list 'lambda (list sym) expr) vals))

Trace

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def trace(fn):
    def traced(n):
        print(f"Calling {fn.__name__} with {n}")
        return fn(n) # n is fn's argument!
    return traced

@trace
def fact(n):
    if n == 0:
        return 1
    else:
        return n * fact(n-1)

print(fact(5))

scheme example, without using macro

with using macro