SWI-Prolog -- Manual

Documentation
- Reference manual
  - Overview
    - The SWI-Prolog syntax
      - ISO Syntax Support
        
        Processor Character Set
        
        Nested comments
        
        Character Escape Syntax
        
        Syntax for non-decimal numbers
        
        Using digit groups in large integers
        
        Rational number syntax
        
        NaN and Infinity floats and their syntax
        
        Force only underscore to introduce a variable
        
        Unicode Prolog source
        
        Singleton variable checking
- Packages

2.16.1.7 NaN and Infinity floats and their syntax

SWI-Prolog supports reading and printing‘special' floating point values according to Proposal for Prolog Standard core update wrt floating point arithmetic by Joachim Schimpf and available in ECLiPSe Prolog. In particular,

Infinity is printed as 1.0Inf or -1.0Inf. Any sequence matching the regular expression [+-]?\sd+[.]\sd+Inf is mapped to plus or minus infinity.
NaN (Not a Number) is printed as 1.xxxNaN, where 1.xxx is the float after replacing the exponent by‘1'. Such numbers are read, resulting in the same NaN. The NaN constant can also be produced using the function nan/0, e.g.,
```
?- A is nan.
A = 1.5NaN.
```

By default SWI-Prolog arithmetic (see section 4.27) follows the ISO standard with describes that floating point operations either produce a normal floating point number or raise an exception. section 4.27.2.4 describes the Prolog flags that can be used to support the IEEE special float values. The ability to create, read and write such values facilitates the exchange of data with languages that can represent the full range of IEEE doubles.