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Function ./2
Availability:Arithmetic function (see is/2)
[deprecated].(+Char,[])
A list of one element evaluates to the character code of this element.123The function is documented as ./2. Using SWI-Prolog v7 and later the actual functor is [|]/2. This implies "a" evaluates to the character code of the letter‘a' (97) using the traditional mapping of double quoted string to a list of character codes. Char is either a valid code point (non-negative integer up to the Prolog flag max_char_code) or a one-character atom. Arithmetic evaluation also translates a string object (see section 5.2) of one character length into the character code for that character. This implies that expression "a" works if the Prolog flag double_quotes is set to one of codes, chars or string.

Getting access to character codes this way originates from DEC10 Prolog. ISO has the 0'a syntax and the predicate char_code/2. Future versions may drop support for X is "a".

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LogicalCaptain said (2021-01-05T13:44:32):0 upvotes 0 0 downvotes
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Does this still exist? How would one use it?

The operator ./2 is now dict dereferencing, so I guess this "feature" is out?

?- X is 0'a.
X = 97.

But:

?- X is .a.
ERROR: Syntax error: Operator expected
ERROR: X is
ERROR: ** here **
ERROR: .a .

?- X is .[a].
ERROR: Syntax error: Operator expected
ERROR: X is
ERROR: ** here **
ERROR: .[a] .

?- .(X,[a]).
ERROR: Unknown procedure: ('.')/2
ERROR:     However, there are definitions for:
ERROR:         ('.')/3
false.