Released Version 3.6
Enhanced the variable storage model to improve support for storing references to objects. For example, consider the following:
(define x (list 'a 'b 'c))
(define y x)
(set-cdr! x 4)
After executing this code, previous versions of husk assigned (a b c) to y. With this release, husk now evaluates y to the expected value of (a . 4).
The more general problem is that certain data types denote a memory location which may be modified by mutator functions such as set-cdr!. This is discussed specifically in section 3.4 Storage Model:
Variables and objects such as pairs, vectors, and strings implicitly denote locations or sequences of locations. A string, for example, denotes as many locations as there are characters in the string. (These locations need not correspond to a full machine word.) A new value may be stored into one of these locations using the string-set! procedure, but the string continues to denote the same locations as before.
Internally husk uses Haskell data types, so the husk model differs slightly from the one in R5RS - references are used instead of individual memory locations. This has implications for the set-car! and set-cdr! special forms, where circular lists and similar low-level optimizations are not possible as Haskell lists are used instead of raw pointers. However, these issues aside, the enhanced storage model is a big step forward to bringing husk’s variable support closer in line to that of other Schemes.