What will __gtf::<u8>(index, GTF_OUTPUT_TYPE) return if there is no output at this index?

fuel · May 17, 2023, 11:07am

Hi guys
I want to add hard predicate outputs check to my predicate
What will __gtf::(index, GTF_OUTPUT_TYPE) return if there is no output at this index?

github.com

FuelLabs/sway/blob/29b1d0c47bfad4573449451ba520ded9e0a26a8a/sway-lib-std/src/outputs.sw#L41


      
          /// The output type for a transaction.
          pub enum Output {
              Coin: (),
              Contract: (),
              Message: (),
              Change: (),
              Variable: (),
          }
          
          
/// Get the type of an output at `index`.
          pub fn output_type(index: u64) -> Output {
              match __gtf::<u8>(index, GTF_OUTPUT_TYPE) {
                  0u8 => Output::Coin,
                  1u8 => Output::Contract,
                  2u8 => Output::Message,
                  3u8 => Output::Change,
                  4u8 => Output::Variable,
                  _ => revert(0),
              }
          }

Voxelot · May 17, 2023, 5:41pm

Now that looping is enabled in predicates, you can first check the number of outputs using GTF_SCRIPT_OUTPUTS_COUNT and then iterate over the correct number of outputs. According to the spec, the predicate will fail if the index is out of bounds:

The value of $rB results in an out of bounds access for variable-length fields

The input or output type does not match (OutputChange and OutputVariable count as OutputCoin)

https://github.com/FuelLabs/fuel-specs/blob/master/src/vm/instruction_set.md#gtf-get-transaction-fields

fuel · May 17, 2023, 6:18pm

Got it
Thanks a lot <3

system · June 7, 2023, 2:18pm

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What will __gtf::<u8>(index, GTF_OUTPUT_TYPE) return if there is no output at this index?

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