2) Types and Variables

#Types and Variables

##Variables

Any string of uppercase latin letters is a variable identifier, initially not unified to anything. These variables and their possible values are not shared between rules.

? and . are special variables which are the Input and the Output of the current rule they are in.

The input ? can take subscripts (like built-in predicates). ?₍ is the first element of the Input. ?₎ is the last element of the Input. ?₄₂ for example is the 42th element of the Input. => This has been reverted due to a bug

Variables can be unified by using them as inputs or outputs to predicates or by direcly unifying them with other variables. For example, I J will unify I with J (or fail if they cannot be unified).

##Lists

Lists are noted in between square brackets [...] and elements of a list are separated using ,. One can also construct a list using :. For example, :2:3. will unify the Output with the list [Input,2,3]. However, 2,3 will not unify the Input with the list [2,3] ; it will instead unify the Input with 2 and then construct the list [2,3] ([2,3] would have unified the Input with [2,3]).

[] denotes the empty list.

##Strings

Strings are opened and closed with double quotes "...". \ will escape double quotes.

##Integers and Floats

Integers and floats are written using the numbers 0 to 9 as expected. The decimal separator is .. A floating point number must have at least one digit before and after the decimal separator.

Negative numbers are written using an underscore _ preceeding the number. This underscore is called the low minus in Brachylog.