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read_string/3

Module​

Built-in predicate.

Description​

Reads characters from Stream and unifies String with an atom containing the text read. Length is unified with the number of UTF-8 bytes read. When Length is instantiated to a positive integer, reading stops once at least that many bytes have been read.

Signature​

read_string(+Stream, ?Length, -String) is det

Examples​

Read a text stream into an atom and byte length​

This scenario demonstrates reading all text from a stream while counting UTF-8 bytes.

Here are the steps of the scenario:

  • Given the program:
read_from_echo(Text, Length, String) :-
  open('/v1/dev/echo', read_write, Stream, [type(text)]),
  write(Stream, Text),
  read_string(Stream, Length, String),
  close(Stream).
  • Given the query:
read_from_echo('aé', Length, String).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 5505
answer:
  has_more: false
  variables: ["Length", "String"]
  results:
  - substitutions:
    - variable: Length
      expression: 3
    - variable: String
      expression: "aé"
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