You must have built an Halo2 verifier executable. -> Learn more here.

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Generate the Verifier Contract

After the prover is built, a Solidity verifier contract can be generated with: 
target/release/halo2-simple-circuit solidity -f ./verifier.sol

Generated verifier contract size: 8100
Just ensure you customize the halo2-simple-circuit executable name with your own prover name.

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Example W3bstream Dapp (Halo2)

https://github.com/iotexproject/w3bstream/blob/develop/examples/halo2-circuit/contracts/Halo2Dapp.sol