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header_info/1

Module​

This predicate is provided by chain.pl.

Load this module before using the predicate:

:- consult('/v1/lib/chain.pl').

Description​

Unifies HeaderInfo with the current SDK header info dict exposed by the VFS.

Returned term shape:

header{
  height: Height,
  hash: [Byte],
  time: Time,
  chain_id: ChainID,
  app_hash: [Byte]
}.

where:

  • Height is the current block height.
  • Time is a Unix timestamp in seconds.
  • ChainID is an atom (quoted if needed).
  • Byte is an integer in [0,255].

Signature​

header_info(-HeaderInfo) is det

Examples​

Retrieve current SDK header info​

This scenario demonstrates how to retrieve the current block header information available to the query. The header info contains useful execution context such as the block height, block time, and chain identifier.

Here are the steps of the scenario:

  • Given the query:
consult('/v1/lib/chain.pl'),
header_info(HeaderInfo).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4091
answer:
  has_more: false
  variables: ["HeaderInfo"]
  results:
  - substitutions:
    - variable: HeaderInfo
      expression: >-
        header{app_hash:[],chain_id:'axone-testchain-1',hash:[],height:42,time:1712745867}

Retrieve the block height of the current block​

This scenario demonstrates how to read the current block height from header_info/1.

Here are the steps of the scenario:

  • Given a block with the following header:
height: 100
  • Given the program:
:- consult('/v1/lib/chain.pl').

height(Height) :-
    header_info(Header),
    Height = Header.height.
  • Given the query:
height(Height).
  • When the query is run
  • Then the answer we get is:
height: 100
gas_used: 4173
answer:
  has_more: false
  variables: ["Height"]
  results:
  - substitutions:
    - variable: Height
      expression: "100"

Retrieve the block time of the current block​

This scenario demonstrates how to read the current block time from header_info/1.

Here are the steps of the scenario:

  • Given a block with the following header:
time: 2024-03-04T11:03:36.000Z
  • Given the program:
:- consult('/v1/lib/chain.pl').

time(Time) :-
    header_info(Header),
    Time = Header.time.
  • Given the query:
time(Time).
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4161
answer:
  has_more: false
  variables: ["Time"]
  results:
  - substitutions:
    - variable: Time
      expression: "1709550216"

Evaluate a condition based on block time and height​

This scenario demonstrates how to evaluate a condition based on both block time and block height. Specifically, it checks whether block time is greater than 1709550216 seconds (Monday 4 March 2024 11:03:36 GMT) or block height is greater than 42.

Here are the steps of the scenario:

  • Given a block with the following header:
time: 2024-03-04T11:03:37.000Z
  • Given the program:
:- consult('/v1/lib/chain.pl').

evaluate :-
    header_info(Header),
    (Header.time > 1709550216; Header.height > 42),
    !.
  • Given the query:
evaluate.
  • When the query is run
  • Then the answer we get is:
height: 42
gas_used: 4195
answer:
  has_more: false
  results:
  - substitutions: