Cute idiom from Haskell

I discovered this fun idiom while implementing an operational semantics in Haskell. The monadic syntax allows you to express monadic binding in either of two directions:

x <- expr;

or

expr -> x;

Combine that with a custom operator definition:

infix 4 |-

and now you can write interpreters whose syntax looks like a big-step evaluation judgment:

(|-) :: Env -> Expr -> Eval Value
env |- Lit n = return (Number n)
env |- Var x = case lookup x env of
                 Just v -> return v
                 Nothing -> throwError $ "free variable " ++ x
env |- Abs x e = return (Closure env x e)
env |- App e1 e2 =
    do env |- e1 -> v1;
       env |- e2 -> v2;
       case v1 of
         Closure env' x e ->
             do (x,v2):env' |- e -> v;
                return v;
         Number n -> throwError $ "apply " ++ (show n)