array-element-type

(array-element-type array)

Returns the array's element type — the upgraded one, so the answer is the type the array actually holds rather than the spelling the program used. A packed float array answers double-float or single-float, and a packed unsigned-integer vector (make-array with :element-type '(unsigned-byte 8|16|32) on a rank-1 array, or a concatenate whose result type spells that element type) answers its real (unsigned-byte n) specifier. A string — a vector of characters — answers character.

A general array remembers an element type it was asked for even where no specialized representation exists for it: character and the three (unsigned-byte n) widths above rank 1, and any of them combined with :fill-pointer/:adjustable. The representation degrades, the declared type does not — and the array's unsupplied elements take that type's own zero rather than nil. Like them, bit is remembered -- a bit vector is the general boxed array stamped bit, with no packed bit storage behind it. An element type with nothing to upgrade to (fixnum, integer, a class) upgrades to t. A displaced view owns no storage, so it answers its target's element type, resolved through the whole displacement chain -- which is the view's own declared :element-type, since make-array requires the two to be type-equivalent. A view's answer does not change when the view stops being one: adjust-array and a vector-push-extend that outgrows the view both un-displace it, and the answer is recorded rather than dropped.