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. See mpg-ref-read_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)
  

  then 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]. See mpg-ref-prolog_flag.
• If the end of the current input stream has been reached, then read(X) will cause X to be unified with the atom end_of_file.