pawsplay
Hero
I struggle to see why any of those would be called a "dagger", unless we're going with the most basic "a thing you can stab with" definition which would include a tonfa as long as it's not too thick. In any case, weapon name discussions are philosophical ones so you're not wrong but I feel it's a weird choice.
A sai is pretty much identical to a main-gauche; a hilted weapon with two side-prongs and a stabbing tip.
If a tonfa is a "dagger" then so is a broomstick. But that's not what I'm saying. A sai is a weapon that has a sharp point you stab into someone's body. It has a hilt. It has a point. It has a guard. It's a dagger.
Similar to these European daggers: