And here is such an example

• fun append(nil,l) = l | append(hd:tl,l) = hd::append(tl,l);

val append = fn : ‘a list * ‘a list  -> ‘a list

• All three ‘a type variables must have the same type

• append([1,2],[3]);

[1,2,3] : int list;

• append([“w”,”us”],[”s”]);

[“w”,”us”,”s”] : string list;

• append([1,2],[“s”]);

…type error…

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