Types and modes

### 6.2 Types and modes

The templates part defines, for each argument of the concerned built-in predicate, its mode and type. The mode specifies whether or not the argument must be instantiated when the built-in predicate is called. The mode is encoded with a symbol just before the type. Possible modes are:

- `+`: the argument must be instantiated.
- `-`: the argument must be a variable (will be instantiated if the built-in predicate succeeds).
- `?`: the argument can be instantiated or a variable.

The type of an argument is defined by the following table:

Type | Description
`TYPE_list` | a list whose the type of each element is `TYPE`
`TYPE1_or_TYPE2` | a term whose type is either `TYPE1` or `TYPE2`
`atom` | an atom
`atom_property` | an atom property (section 8.19.11)
`boolean` | the atom `true` or `false`
`byte` | an integer ≥ 0 and ≤ 255
`callable_term` | an atom or a compound term
`character` | a single character atom
`character_code` | an integer ≥ 1 and ≤ 255
`clause` | a clause (fact or rule)
`close_option` | a close option (section 8.10.7)
`compound_term` | a compound term
`evaluable` | an arithmetic expression (section 8.6.1)
`fd_bool_evaluable` | a boolean FD expression (section 9.7.1)
`fd_labeling_option` | an FD labeling option (section 9.9.1)
`fd_evaluable` | an arithmetic FD expression (section 9.6.1)
`fd_variable` | an FD variable
`flag` | a Prolog flag (section 8.22.1)
`float` | a floating point number
`head` | a head of a clause (atom or compound term)
`integer` | an integer
`in_byte` | an integer ≥ 0 and ≤ 255 or `-1` (for the end-of-file)
`in_character` | a single character atom or the atom `end_of_file` (for the end-of-file)
`in_character_code` | an integer ≥ 1 and ≤ 255 or `-1` (for the end-of-file)
`io_mode` | an atom in: `read`, `write` or `append`
`list` | the empty list `[]` or a non-empty list `[_|_]`
`nonvar` | any term that is not a variable
`number` | an integer or a floating point number
`operator_specifier` | an operator specifier (section 8.14.10)
`os_file_property` | an operating system file property (section 8.27.11)
`predicate_indicator` | a term `Name/Arity` where `Name` is an atom and `Arity` an integer ≥ 0. A callable term can be given if the `strict_iso` Prolog flag is switched off (section 8.22.1)
`predicate_property` | a predicate property (section 8.8.2)
`read_option` | a read option (section 8.14.1)
`socket_address` | a term of the form `’AF_UNIX’(A)` or `’AF_INET’(A,N)` where `A` is an atom and `N` an integer
`socket_domain` | an atom in: `’AF_UNIX’` or `’AF_INET’`
`source_sink` | an atom identifying a source or a sink
`stream` | a stream-term: a term of the form `’$stream’(N)` where `N` is an integer ≥ 0
`stream_option` | a stream option (section 8.10.6)
`stream_or_alias` | a stream-term or an alias (atom)
`stream_position` | a stream position: a term `’$stream_position’(I1, I2, I3, I4)` where `I1`, `I2`, `I3` and `I4` are integers
`stream_property` | a stream property (section 8.10.10)
`stream_seek_method` | an atom in: `bof`, `current` or `eof`
`term` | any term
`var_binding_option` | a variable binding option (section 8.5.3)
`write_option` | a write option (section 8.14.6)

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