What is a type error?

[Matthias Blume]

Pascal Costanza <pc at p-cos.net> writes:

> Chris Smith wrote:
>
>> While this effort to salvage the term "type error" in dynamic
>> languages is interesting, I fear it will fail.  Either we'll all
>> have to admit that "type" in the dynamic sense is a psychological
>> concept with no precise technical definition (as was at least hinted
>> by Anton's post earlier, whether intentionally or not) or someone is
>> going to have to propose a technical meaning that makes sense,
>> independently of what is meant by "type" in a static system.
>
> What about this: You get a type error when the program attempts to
> invoke an operation on values that are not appropriate for this
> operation.
>
> Examples: adding numbers to strings; determining the string-length of
> a number; applying a function on the wrong number of parameters;
> applying a non-function; accessing an array with out-of-bound indexes;
> etc.

Yes, the phrase "runtime type error" is actually a misnomer.  What one
usually means by that is a situation where the operational semantics
is "stuck", i.e., where the program, while not yet arrived at what's
considered a "result", cannot make any progress because the current
configuration does not match any of the rules of the dynamic
semantics.

The reason why we call this a "type error" is that such situations are
precisely the ones we want to statically rule out using sound static
type systems.  So it is a "type error" in the sense that the static
semantics was not strong enough to rule it out.

> Sending a message to an object that does not understand that message
> is a type error. The "message not understood" machinery can be seen
> either as a way to escape from this type error in case it occurs and
> allow the program to still do something useful, or to actually remove
> (some) potential type errors.

I disagree with this.  If the program keeps running in a defined way,
then it is not what I would call a type error.  It definitely is not
an error in the sense I described above.