The term latent typing is used to describe a computer language, such as Scheme, for which you cannot, in general, simply look at a program’s source code and determine what type of data will be associated with a particular variable, or with the result of a particular expression.
Sometimes, of course, you can tell from the code what the type of
an expression will be. If you have a line in your program that sets the
x to the numeric value 1, you can be certain that,
immediately after that line has executed (and in the absence of multiple
x has the numeric value 1. Or if you write a procedure
that is designed to concatenate two strings, it is likely that the rest
of your application will always invoke this procedure with two string
parameters, and quite probable that the procedure would go wrong in some
way if it was ever invoked with parameters that were not both strings.
Nevertheless, the point is that there is nothing in Scheme which
requires the procedure parameters always to be strings, or
always to hold a numeric value, and there is no way of declaring in your
program that such constraints should always be obeyed. In the same
vein, there is no way to declare the expected type of a procedure’s
Instead, the types of variables and expressions are only known – in general – at run time. If you need to check at some point that a value has the expected type, Scheme provides run time procedures that you can invoke to do so. But equally, it can be perfectly valid for two separate invocations of the same procedure to specify arguments with different types, and to return values with different types.
The next subsection explains what this means in practice, for the ways that Scheme programs use data types, values and variables.