racket - how to make predicates function? in scheme -

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I am trying to take action on future

Datatype definition 

  (define-datatype expression expression? (Num number?)) (Var-exp (var symbol?)) (Zero? -exp (exp1 expression?)) (Diff-exp (exp1 expression?) (Exp2 expression?)))   
 If the argument is # 2 (straight: const-exp 7) then print #t  
 if argument # 2 (struct: var - Exp x) Then print # f  
  (const-exp define? ...)   
 If you help me, then this is respect  
  
 
 
 So my guess is that you are reading and probably use it as your environment .  
 If this is true, you should probably use it here:  
  (define-const-exp? E) (case expression (const-exp ( num) #t) (else #f))   
 then  
  & gt; (Cost-exp? (Constant-Express 7)) # T & gt; (Consta-exp? (Vir XAP 'X)) #F

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