Question

Sequence (toBlock $ VarDeclAssign "foo" (JBool False)) (Return $ JBool True)

This is the only way I can do it at the moment. I tried a foldl but EmptyBlock has a different type so it wouldn’t build.

Can I use do notation or something here instead?

Answer 1

Looking at the documentation, the problem is that the types of EmptyBlock and Sequence are

EmptyBlock :: Block ()   
Sequence :: Block () -> Stmt t -> Block t

Most statements are of type Stmt (), except for Return :: Expr t -> Stmt t. From what I can tell, this is to ensure that there can only be a single return statement in a block, and it must be at the end.

The reason why foldl Sequence EmptyBlock didn’t work for you is that foldl has the type (a -> b -> a) -> a -> [b] -> a, and if you look at the first argument, it’s clear that Sequence can only be used if you pick t = () so that a = Block () and b = Stmt ().

This means that you can use foldl to combine all statements except for the Return, which you have to manually insert at the end. Hopefully, that shouldn’t be too inconvenient.