Never got too far into Haskell despite reading up on it several times and writing a little code, but adding constraints does sound exactly like this. One of the

Never got too far into Haskell despite reading up on it several times and writing a little code, but adding constraints does sound exactly like this. One of the prototype P6 implementations was written in Haskell, so P6 stole some features. The only thing I'm not sure about is Haskell does this at compile-time and P6 does this at run-time I believe.