September 12, 2020
Here is a small example of aggressive refactoring in Haskell. I am in the codebase of a compiler for a small smart contract language, and I'm replacing occurrences of
Integer with occurrences of
Word256, since the code generator in question targets Ethereum which has 256-bit word sizes.
The compiler was originally made with
Integer, and in some cases its very own
Word256 which was made in a crude way.
Integer because this is a very convenient data type in Haskell, even though arbitrary precision in this case is actually wrong, and the crude
Word256 because it seemed simpler to make this type rather than import a library.
But now this is no longer convenient, so I've decided to use
Word256 of the
data-dword library since that's what Hevm uses. There are alternatives:
wide-word, but the important part for now is interoperability.
And here is the type error I'd like to show:
src/Lira/Backends/Evm/EvmCompiler.hs:695:22: error: • Couldn't match expected type ‘Integer’ with actual type ‘Data.DoubleWord.Word256’ • In the first argument of ‘push’, namely ‘i’ In the expression: push i In the expression: [push i] | 695 | IntVal i -> [push i] | ^
IntVal i now contains a
push was designed for
Integer, the types no longer align. Looking at
push :: Integer -> EvmOpcode push = PUSHN . words' where words' :: Integer -> [Word8] words' i | i < 256 = [fromIntegral i] words' i = words' (i `div` 256) ++ [fromIntegral $ i `mod` 256]
There is no logic in this function that prevents it from operating on
Word256 rather than
Integer. In fact, rather than change the function, just loosen the type signature:
-push :: Integer -> EvmOpcode +push :: Integral i => i -> EvmOpcode push = PUSHN . words' where - words' :: Integer -> [Word8] + words' :: Integral i => i -> [Word8] words' i | i < 256 = [fromIntegral i] words' i = words' (i `div` 256) ++ [fromIntegral $ i `mod` 256]
Doing an aggressive refactor like changing a base type in the codebase requires hundreds of changes. But when their nature is like this, I can safely leave this change be and proceed to the next type error.