fix log

lectures/lecture05.md

@@ -65,6 +65,9 @@ before the compilation by a macro is possible.
 ```scheme
 (define-syntax-rule (macro-if c a b)
   (my-if c (thunk a) (thunk b)))
+
+(define (my-if c a b)
+  (if c (a) (b)))
 ```
 The macro defines a syntax rule whose pattern is `(macro-if c a b)`. Racket
 searches for a portion of AST being a list of length $4$ starting with a symbol

lectures/lecture05.rkt

@@ -1,44 +1,46 @@
 #lang racket
 
-;;; Lecture 6 - mutable data
-
-; set!
-(define x 1)
-x
-(set! x 2)
-x
-(set! x '(a b c))
-x
-
-(define (make-counter)
-  (define cnt 0)
-  (lambda ()
-    (set! cnt (+ cnt 1))
-    cnt))
-
-; pseudorandom number generator
-(define random
-  (let ([a 69069]
-        [b 1]
-        [m (expt 2 32)]
-        [seed 20210323])
-    (lambda args
-      (if (null? args)
-          (begin
-            (set! seed
-                  (modulo (+ (* a seed) b) m))
-            (exact->inexact (/ seed m)))
-          (set! seed (car args))))))
-
-(random)
-(random 666)
-
-; vectors
-(define v (vector 1 2 3))
-v
-(vector-ref v 2)
-(vector-set! v 2 "hi")
-v
-(make-vector 4 'a)
-(define v2 #('a 'b 'c))  ; v2 is immutable
-;(vector-set! v2 1 'd)
+;;; Lecture 5 - Streams and macros
+;;; Lazy evaluation
+
+; lazy evaluation of arguments via thunks
+(define (my-if c a b)
+  (if c a b))
+
+(define (my-lazy-if c a b) (if c (a) (b)))
+(my-lazy-if #t (thunk 0) (thunk (/ 1 0)))
+
+
+;;; Macros
+
+; lazy if
+(define-syntax-rule (macro-if c a b)
+  (my-lazy-if c (thunk a) (thunk b)))
+
+(macro-if (< 0 1) 'then (/ 1 0))
+
+; list comprehension
+(define-syntax list-comp
+  (syntax-rules (: <- if)
+    [(list-comp <expr> : <id> <- <lst>)
+     (map (lambda (<id>) <expr>) <lst>)]
+
+    [(list-comp <expr> : <id> <- <lst> if <cond>)
+     (map (lambda (<id>) <expr>)
+          (filter (lambda (<id>) <cond>) <lst>))]))
+
+(list-comp (+ x 2) : x <- (range 10) if (>= 3 x))
+
+; log macro
+(define ((log-rt f fsymb) . xs)
+  (let ([val (apply f xs)])
+    (printf "Function: ~a\n" fsymb)
+    (printf "Arguments: ~a\n" xs)
+    (printf "Result: ~a\n" val)
+    val))
+
+(define-syntax-rule (log f) (log-rt f 'f))
+
+(map (log *) (range 5) (range 5))
+(foldl (log cons) '() (range 3))
+