#lang scheme
(Define-syntax My-when
(Syntax-rules ()
[(_ Pred body ...)
(if pred (begin body ...) (void)) ] ) )
(My-when
(= 2 1)
(Display 1)
(Display 2))
(Define-syntax My-let
(Syntax-rules ()
[(My-let ([var exp] ...)
Body ...)
((Lambda (var ...)
Body ...) Exp ...) ] ) )
(My-let
([a 1]
[B 2])
(Display a)
(Display B))
(Define-syntax my-let*
(Syntax-rules ()
[(my-let* ([var exp]) body ...)
(My-let ([var exp]) body ...) ]
[(my-let* ([var exp]
[Var1 EXP1] ...) Body ...)
(My-let ([var exp])
(My-let* ([var1 exp1] ...)
Body ...) ) ] ) )
(Define-syntax my-for
(Syntax-rules ()
[(My-for (var from to) body ...)
(Let loops ([var from])
(My-when
(< var to)
Body ...
(Loop (+ var 1))) ) ] ) )
(my-for
(I 0 10)
(display "a")
(display "B"))
;;; A more interesting usage.
(Define-syntax Loop
(Syntax-rules (begin end from to)
[Loop <var> from <min> to <max> start body ... end]
(Let Loop1 ([<var> <min>])
(Cond
[(< <var> <max>) body ... (LOOP1 (+ 1 <var>))]
[Else (void)]) ) ] ) )
(Loop i from 1 to 10
Begin
(Display 1)
(newline)
(Display 2)
(newline)
End)
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Use of macros in scheme
