Lisp - Variable Capture and Hygienic Macros

A Variable Capture is a problem which occurs when a Macro introduces a variable of same name as that passed as an argument leading to unintentional result. A macros facing such variable capture is termed as unhygienic macro. LISP provides multiple ways to sort variable capture problem. In this chapter, we'll explore variable capture and writing hygienic macros with examples.

Example - Variable Capture in Macro

main.lisp

; define a macro to swap two numbers 
(defmacro swap (x y)
   `(let ((temp ,x))  ; variable capturing occurs when x is passed as temp
      (setf ,x ,y)
      (setf ,y temp)))
     
; define two variables temp and my-var
(let ((temp 1) (my-var 2))
      (swap temp my-var) ; swap macro unintentionally uses local temp over passed argument temp
      (print temp)       ; temp is not modified
      (print my-var))    ; my-var is not modified

Output

When you execute the code, it returns the following result −

1
2

Explanation

• (defmacro swap (x y) − macro swap is defined using defmacro with arguments x and y

• `(...,) − a template is created using backquote and using comma, value of x will be inserted in the template.

• (let ((temp ,x)) − a local variable temp is created, with value of x

• (setf ,x ,y) − value of x is replaced with value of y

• (setf ,y ,temp) − value of y is replaced with value of temp thus swapping the numbers

• (let ((temp 1) (my-var 2)) − local variable temp and my-var are initialized with 1 and 2 respectively.

• (swap temp my-var) − macro will replace the code with temp and var as passed arguments. As temp is already present in local scope, passed argument is not used.

• (print temp) − prints the unmodified value of temp variable.

• (print my-var) − prints the unmodified value of my-var variable.

Example - Hygienic Macro

In order to solve variable capture problem, we can use gensym function to generate the unique symbol for the local variable of macro. In above code we can replace `(let ((temp ,x)) with (let ((temp (gensym))) `(let ((,temp ,x)) so that temp variable in renamed to a unique name in macro as shown in example below.

main.lisp

; define a macro to swap two numbers 
(defmacro swap (x y)
   (let ((temp (gensym))) ; create a unique variable
      `(let ((,temp ,x))  ; insert the unique variable and value of x
         (setf ,x ,y)
         (setf ,y ,temp))))
     
(let ((temp 1) (my-var 2)) 
   (swap temp my-var) ; swap macro correctly uses passed temp argument
   (print temp) 
   (print my-var))

Output

When you execute the code, it returns the following result −

2
1

Explanation

• (defmacro swap (x y) − macro swap is defined using defmacro with arguments x and y

• (let ((temp (gensym)))− creates a temp variable appended with a unique symbol say 123, as temp123.

• `(...,) − a template is created using backquote and using comma, value of x will be inserted in the template.

• (let ((,temp ,x)) − a variable temp is inserted a new variable name created using gensym, with value of x

• (setf ,x ,y) − value of x is replaced with value of y

• (setf ,y ,temp) − value of y is replaced with value of temp thus swapping the numbers

• (let ((temp 1) (my-var 2)) − local variable temp and my-var are initialized with 1 and 2 respectively.

• (swap temp my-var) − macro will replace the code with temp and var as passed arguments. As temp is not present in local scope now, passed argument is used.

• (print temp) − prints the modified value of temp variable.

• (print my-var) − prints the modified value of my-var variable.