[1] =:= 1

General
1

Hi,

Why does this goal evaluate to true ?

?- [1] =:= 1
true
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2

I suspect it is this one. The docs talk about chars and one-character atoms but I am guessing that using an integer is also accepted. You can also do this:

?- ['1'] =:= 49.
true.

?- `1` =:= 49.
true.

?- "a" =:= 97.
true.
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3

Ok, it is a suprising behavior
thank you Boris for your answer

4

Sicstus docs says:

[X] A list of just one number X evaluates to X. Since a quoted string is just a list of integers, this allows a quoted character to be used in place of its character code; e.g. "A" behaves within arithmetic expressions as the integer 65.

Why does this goal evaluate to true ?

I suppose that this is informally a convention that the early Prolog implementations adopted.

Note that quoted strings may work differently in SWI

5

Yes, “a” maps to a string, but arithmetic evaluation evaluates a string with a single character as the character code of that character, so A is "a" still gets you the character code for a. Modern code should use 0'a instead. That is handled by the parser and read as 97.

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