abi decoding of simple custom errors

Only supports errors without arguments for now

ethers/errors.nim

@@ -0,0 +1,17 @@
+import pkg/contractabi/selector
+import ./basics
+
+type SolidityError* = object of EthersError
+
+{.push raises:[].}
+
+template errors*(types) {.pragma.}
+
+func decode*[E: SolidityError](_: type E, data: seq[byte]): ?!(ref E) =
+  const name = $E
+  const selector = selector(name, typeof(()))
+  if data.len < 4:
+    return failure "unable to decode " & name & ": signature too short"
+  if selector.toArray[0..<4] != data[0..<4]:
+    return failure "unable to decode " & name & ": signature doesn't match"
+  success (ref E)()

testmodule/test.nim

@@ -7,5 +7,6 @@ import ./testWallet
 import ./testTesting
 import ./testErc20
 import ./testGasEstimation
+import ./testErrorDecoding
 
 {.warning[UnusedImport]:off.}

testmodule/testErrorDecoding.nim

@@ -0,0 +1,30 @@
+import std/unittest
+import pkg/questionable/results
+import pkg/ethers/errors
+
+suite "Decoding of custom errors":
+
+  type SimpleError = object of SolidityError
+
+  test "decodes a simple error":
+    let decoded = SimpleError.decode(@[0xc2'u8, 0xbb, 0x94, 0x7c])
+    check decoded is ?!(ref SimpleError)
+    check decoded.isSuccess
+    check (!decoded) != nil
+
+  test "returns failure when decoding fails":
+    let invalid = @[0xc2'u8, 0xbb, 0x94, 0x0] # last byte is wrong
+    let decoded = SimpleError.decode(invalid)
+    check decoded.isFailure
+
+  test "returns failure when data is less than 4 bytes":
+    let invalid = @[0xc2'u8, 0xbb, 0x94]
+    let decoded = SimpleError.decode(invalid)
+    check decoded.isFailure
+
+  test "decoding only works for SolidityErrors":
+    type InvalidError = ref object of CatchableError
+    const works = compiles:
+      InvalidError.decode(@[0x1'u8, 0x2, 0x3, 0x4])
+    check not works
+