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Why is there no error when defining a function using these overloaded operators?

I am reading Programming with Standard ML by Colin Myers, Chris Clack, and Ellen Poon. In § 2.4.1 Restricted polymorphic functions:

Unlike the operators = and <>, the other comparison operators (>, < etc.) are overloaded; they are not restricted polymorphic and therefore the following definition fails:

- fun wrong_ordered (x, y, z)
     = (x < y) andalso (y < z);
Error: overloaded variable "<" cannot be resolved

This failure occurs because SML does not know the types of x, y and z.

When I entered that definition into SML/NJ 110.99.2 and Poly/ML 5.9, I did not get any error message. The definition succeeds:

- fun wrong_ordered (x, y, z) = (x < y) andalso (y < z);
val wrong_ordered = fn : int * int * int -> bool

Why did I not get the error that the authors said I would be getting?

Solution

  • The book predates SML'97. With that language version, type resolution for overloaded operators was refined. In particular, overloaded types now default to int when they cannot be resolved by context.