xの型がT
y = Foo(x)なら
yの型はFoo(T)

というシステムでいいと思うの

Length ::= Float
Angle ::= Float
Point ::= Tuple(Float, Float)
Triangle ::=
Triangle(Length, Length, Length)
| Triangle(Length, Length, Angle)
| Triangle(Length, Angle, Angle)
| Triangle(Point, Point)