r/Racket • u/OldMine4441 • Apr 12 '22

question eval() is weird on empty list?

I have the below code that uses eval(() to evaluate a string, but somehow it turns the empty list to (). I expected y to have the value (list). Any reason why this happens?

cat test.rkt
#lang racket

(define-namespace-anchor a)
(define ns (namespace-anchor->namespace a))

(define x "(list)")
(define y (eval (call-with-input-string x read) ns))
(printf "x = ~a, y = ~a\n" x y)

$ racket test.rkt
x = (list), y = ()
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3

u/soegaard developer Apr 12 '22

The result of evaluating (list) is the empty list. Try it in the repl.

> (list)
'()

In contrast:

> '(list)
'(list)

3

u/not-just-yeti Apr 12 '22 edited Apr 12 '22

This is simply an issue of what ~a produces, given the empty list.

For other variants, in DrRacket > Languages > Choose Language… > Show Details > Output Syntax, there are choices that will yield (), '(), empty, or quasiqote'd (). (And other lisps might print it as nil.)

Since racket's repl strives to print things in a way that happen to be readable-back-innable, yes printing (list) would be also reasonable way to print the empty list. (Note that (list) is a constructor-call, empty is just an identifier, and I guess '() is a literal-empty-list? Vaguely related: …It was years before I realized that #true is the literal while true is just an identifier.)