Scryer-prolog: syntax_error(incomplete_reduction) for backslash

[notoria]

The following happens:

?- A = (\).
   A = (\).
?- A = '\\' .
caught: error(syntax_error(incomplete_reduction),read_term/3:1)
?-

Expected the second query to succeed.

[triska]

We have:

?- current_op(Pri, Fix, \).
   Pri = 200, Fix = fy
;  false.

So, \ is a predefined prefix operator.

6.3.1.3 Atoms
...
          term = atom ;
Abstract: a      a
Priority: 1201
Condition:a is an operator

An atom which is an operator shall not be the immediate
operand (3.120) of an operator. The priority of a term
consisting of an operator is therefore given the priority
1201 rather than the normal 0.

We also have for example:

?- X = + .
caught: error(syntax_error(incomplete_reduction),read_term/3:1)

For the same reason as above.

Write (+) and (\\) etc., i.e. enclose the term in round brackets, to give it priority 0:

          term = open ct, term, close ;
Abstract: a               a
Priority: 0               1201

For example:

?- X = (+).
   X = (+).

[notoria]

Thank you, so this also fails ?- X = is., closing.

[triska]

Yes exactly!

This is why I always write (#>)/2, (#=)/2 etc. to refer to these predicates: To make them valid Prolog terms when #> and #= are operators!

Scryer Prolog's syntactic strictness and ISO conformance is an amazing feature: It means that we can use the system to test what is portable syntax, and what is not. It's a huge attraction of the system.