4.6.3.1 Reading Terms: The "Read" Predicates

The “Read” predicates are

• read(-Term)
• read(+Stream, -Term)
• read_term(-Term, +Options)
• read_term(+Stream, -Term, +Options)

read_term/[2,3] offers many options to return extra information about the term.

When Prolog reads a term from the current input stream the following conditions must hold:

• The term must be followed by a full-stop. See ref-syn-syn-ove. The full-stop is removed from the input stream but is not a part of the term that is read.

  read/[1,2] does not terminate until the full-stop is encountered. Thus, if you type at top level

            | ?- read(X)
  

  you will keep getting prompts (first ‘|: ’, and five spaces thereafter) every time you type <RET>, but nothing else will happen, whatever you type, until you type a full-stop.

• The term is read with respect to current operator declarations. See ref-syn-ops for a discussion of operators.
• When a syntax error is encountered, an error message is printed and then the “read” predicate tries again, starting immediately after the full-stop that terminated the erroneous term. That is, it does not fail on a syntax error, but perseveres until it eventually manages to read a term. This behavior can be changed with prolog_flag/3 or using read_term/[2,3].
• If the end of the current input stream has been reached, read(X) will cause X to be unified with the atom end_of_file.