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custom let with local variables from list (defmacro)

Adrian Nistor

Hi,

I'm playing around with defmacro (as a defmacro beginner) and I can not succeed :(

I have a list of strings:

myVarList = list( "a" "b" "c" )   ;;any number of parameters, not necessary 3 !

[ or: myVarSymbolList = list( 'a 'b 'c)   ]

I want to define a custom let where my string would be local variable, and I would like to use it like this

myCustomLet( myVarList
  ;; here starts my normal code having a, b and c as local variables
  a = 1    b = 2    c = 3
)

I don't know how to do it properly :(

I tried like this:

(defmacro customLetWithLocalVars (listOfVars @rest code )
    `(let  ,listOfVars ,@code )
)

But it is working only like this:
customLetWithLocalVars(    (a b c)  a=1 b=2 c=3)

and NOT LIKE THIS:

customLetWithLocalVars(   myVarSymbolList   a=1 b=2 c=3 )

:(

Do you have any hints?

  • Adrian,

    You can't do this with a macro. The value of myVarSymbolList would only be known at run time, not at the time the macro is expanded. You'd have to do something like this:

    (defun customLetWithLocalVarsInt (listOfVars code)
      (eval `(let ,(mapcar 'concat listOfVars) ,@code)))

    (defmacro customLetWithLocalVars (listOfVars @rest code)
      `(customLetWithLocalVarsInt ,listOfVars ',code))

    Uses run-time evaluation (eval), which isn't brilliant - but I don't see how else you could do it. Seems a slightly unusual thing to want to do to me.

    The above example will work with lists of strings or lists of symbols. The macro is just so that you don't have to quote the code.

    Regards,

    Andrew.

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