Web3.Contract.Call

Type-safe contract read calls (eth_call).

Build a read call, encode it for the JS port, decode the response. The return type is parameterized so your decoder is checked at compile time.

import Web3.Contract.Call as Call
import Web3.Abi.Decode as Decode
import Web3.Abi.Encode as Encode


-- Read a uint256 return value
totalSupply : T.Address -> Call.ReadCall BigInt
totalSupply token =
    Call.readCall
        { contract = token
        , method = "totalSupply()"
        , args = []
        , decoder = Decode.uint256
        , id = "total-supply"
        }


-- Simulate a write to catch reverts before broadcasting
simulate : T.Address -> BigInt -> T.Address -> Call.ReadCall Bool
simulate router amount caller =
    Call.readCall
        { contract = router
        , method = "buy(uint256)"
        , args = [ Encode.uint256 amount ]
        , decoder = Decode.bool
        , id = "sim-buy"
        }
        |> Call.withFrom caller


-- Send via port
web3Cmd (Call.encode (totalSupply tokenAddress))


-- Decode response
result = D.decodeValue (Call.responseDecoder myCall) incoming

Related modules: Web3.Multicall for batching multiple reads into one RPC call; Web3.Contract.Send for write calls.

type ReadCall a

A read-only contract call with a typed return value.

readCall :

{ contract : Address

, method : String

, args : List Value

, decoder : Decoder a

, id : String

}

-> ReadCall a

Create a read call. The id is echoed back in the response so you can match responses to requests when multiple calls are in flight.

readCallRaw :

{ contract : Address

, data : String

, decoder : Decoder a

, id : String

}

-> ReadCall a

Create a read call from pre-built hex calldata — the result of Web3.Abi.Calldata.calldata. The JS port bridge sends data directly without re-encoding, so the wire path is entirely pure-Elm: no method-name string, no arg encoding outside of Elm.

-- balanceOf(address) — selector baked at codegen time
balanceOf : T.Address -> T.Address -> ReadCall BigInt
balanceOf contract holder =
    readCallRaw
        { contract = contract
        , data = Calldata.calldata "70a08231" [ Calldata.address holder ]
        , decoder = AbiDecode.uint256
        , id = "balanceOf"
        }

The data argument must be a complete "0x…" calldata string including the 4-byte function selector.

withBlock : BlockNumber -> ReadCall a -> ReadCall a

Set the block number for the call.

withFrom : Address -> ReadCall a -> ReadCall a

Add a from address — turns eth_call into a simulation of a write. Catches reverts without broadcasting.

simulateBuy tokenAddress amount userAddress
    |> withFrom userAddress
    |> encode
    |> web3Cmd

encode : ReadCall a -> Value

Encode a read call for the JS port.

responseDecoder : ReadCall a -> Decoder a

Get the response decoder for a read call.

intrepidshape/elm-web3/1.4.4

Web3.Contract.Call

Modules