10.6 Deferred and Lazy Evaluation

Sometimes it is useful to delay the evaluation of an expression, for example if you want to avoid performing a time-consuming calculation if it turns out that the result is not needed in the future of the program. The thunk library provides the following functions and macros to support such deferred evaluation:

Macro: thunk-delay forms…

Return a thunk for evaluating the forms. A thunk is a closure (see Closures) that inherits the lexical environment of the thunk-delay call. Using this macro requires lexical-binding.

Function: thunk-force thunk

Force thunk to perform the evaluation of the forms specified in the thunk-delay that created the thunk. The result of the evaluation of the last form is returned. The thunk also “remembers” that it has been forced: Any further calls of thunk-force with the same thunk will just return the same result without evaluating the forms again.

Macro: thunk-let (bindings…) forms…

This macro is analogous to let but creates “lazy” variable bindings. Any binding has the form (symbol value-form). Unlike let, the evaluation of any value-form is deferred until the binding of the according symbol is used for the first time when evaluating the forms. Any value-form is evaluated at most once. Using this macro requires lexical-binding.

Example:

(defun f (number)
  (thunk-let ((derived-number
              (progn (message "Calculating 1 plus 2 times %d" number)
                     (1+ (* 2 number)))))
    (if (> number 10)
        derived-number
      number)))

(f 5)
⇒ 5

(f 12)
⊣ Calculating 1 plus 2 times 12
⇒ 25

Because of the special nature of lazily bound variables, it is an error to set them (e.g. with setq).

Macro: thunk-let* (bindings…) forms…

This is like thunk-let but any expression in bindings is allowed to refer to preceding bindings in this thunk-let* form. Using this macro requires lexical-binding.

(thunk-let* ((x (prog2 (message "Calculating x...")
                    (+ 1 1)
                  (message "Finished calculating x")))
             (y (prog2 (message "Calculating y...")
                    (+ x 1)
                  (message "Finished calculating y")))
             (z (prog2 (message "Calculating z...")
                    (+ y 1)
                  (message "Finished calculating z")))
             (a (prog2 (message "Calculating a...")
                    (+ z 1)
                  (message "Finished calculating a"))))
  (* z x))

⊣ Calculating z...
⊣ Calculating y...
⊣ Calculating x...
⊣ Finished calculating x
⊣ Finished calculating y
⊣ Finished calculating z
⇒ 8

thunk-let and thunk-let* use thunks implicitly: their expansion creates helper symbols and binds them to thunks wrapping the binding expressions. All references to the original variables in the body forms are then replaced by an expression that calls thunk-force with the according helper variable as the argument. So, any code using thunk-let or thunk-let* could be rewritten to use thunks, but in many cases using these macros results in nicer code than using thunks explicitly.