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Version: v11.0.0

current_output/1

Description​

current_output/1 is a predicate that unifies the given term with the current output stream.

Signature​

current_output(-Stream) is det

where:

  • Stream represents the current output stream.

This predicate connects to the default output stream available for user interactions, allowing the user to perform write operations.

The outcome of the stream's content throughout the execution of a query is provided as a string within the user_output field in the query's response. However, it's important to note that the maximum length of the output is constrained by the max_query_output_size setting, meaning only the final max_query_output_size bytes (not characters) of the output are included in the response.

Examples​

Write a char to the current output​

This scenario demonstrates how to write a character to the current output, and get the content in the response of the request.

Here are the steps of the scenario:

  • Given the module configuration:
{
  "limits": {
    "max_user_output_size": 5
  }
}
  • Given the program:
write_char_to_user_output(C) :-
    current_output(UserStream), % get the current output stream
    put_char(UserStream, C).    % write the char to the user stream
  • Given the query:
write_char_to_user_output(x).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4043
answer:
  has_more: false
  variables:
  results:
  - substitutions:
user_output: |
  x

Write characters to the current output (without limit)​

This scenario demonstrates how to write some characters to the current output, and get the content in the response of the request. This is helpful for debugging purposes.

Here are the steps of the scenario:

  • Given the module configuration:
{
  "limits": {
    "max_user_output_size": 15
  }
}
  • Given the program:
log_message(Message) :-
    current_output(UserStream), % get the current output stream
    write(UserStream, Message), % write the message to the user stream
    put_char(UserStream, '\n').
  • Given the query:
log_message('Hello world!').
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4045
answer:
  has_more: false
  variables:
  results:
  - substitutions:
user_output: |
  Hello world!

Write characters to the current output (with limit)​

This scenario demonstrates the process of writing characters to the current user output, with a limit configured in the logic module. So if the message is longer than this limit, the output will be truncated.

Here are the steps of the scenario:

  • Given the module configuration:
{
  "limits": {
    "max_user_output_size": 5
  }
}
  • Given the program:
log_message(Message) :-
    current_output(UserStream), % get the current output stream
    write(UserStream, Message). % write the message to the user stream
  • Given the query:
log_message('Hello world!').
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4044
answer:
  has_more: false
  variables:
  results:
  - substitutions:
user_output: |
  orld!

Write UTF-8 character to the current output (with limit)​

This scenario illustrates the impact of UTF-8 characters on output limits measured in bytes, not character count. Characters such as emojis require more space; for example, the wizard emoji (🧙) occupies 4 bytes, effectively counting as four units. As a result, the limit is reached more quickly with these characters, which means that the number of characters in the user output is less than expected.

Here are the steps of the scenario:

  • Given the module configuration:
{
  "limits": {
    "max_user_output_size": 5
  }
}
  • Given the program:
log_message([]).
log_message([H|T]) :-
    current_output(UserStream),
    put_char(UserStream, H),
    log_message(T).
  • Given the query:
log_message("Hello 🧙!").
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4065
answer:
  has_more: false
  variables:
  results:
  - substitutions:
user_output: "🧙!"