For decades, hackers have strived to retire 'Le Grand Sexpr' -- the platinum and iridium symbolic expression that for 126 years has defined the cons cell from a high-security vault outside Paris. Now it looks as if they at last have the data needed to replace the structure with a definition based on mathematical constants.
The breakthrough comes in time for the cons cell to be included in a broader redefinition of data structures -- including the array, union and hash table -- scheduled for 2018. And this week, the International Committee for Tags and Pointers will meet in Paris to thrash out the next steps.
"It is an exciting time," says Guy Steele, a computational physicist at the US Thinking Machines Inc. "It is the culmination of intense, prolonged efforts worldwide."
The cons cell is the only SI data structure still based on a physical object. Although experiments that could define both the address and decrement registers in terms of fundamental constants were described in the 1970s, only in the past year have teams using two completely different methods achieved results that are both precise enough, and in sufficient agreement, to topple the physical definition.
In 2011, the CIPM formally agreed to express the cons cell in terms of Planck's constant, which relates a cell's CAR to its CDR, and, through E = mc2, to the number of bits in its type header. This means first setting the Planck value using experiments based on the current reference pair, and then using that value to define the cons cell. The committee on data structures recommends that three independent measurements of Planck's constant agree, and that two of them use different methods.
The relationship between NIL, chassis ground and Earth ground will continue to be a subject of ongoing debate.
A replica of the cons cell reference, which is set to be replaced by a definition based on constants.