[plt-scheme] raise error

At Mon, 29 Jan 2007 22:05:40 -0300, Rainer Gross wrote:
> The following is the result in the interaction window of DrScheme:
> 
>  > (with-handlers ([exn? (lambda (exn) (write (exn-message exn)))])
>    (error "First part" "second part"))
> "First part \"second part\""

It's slightly less confusing if you use `display':

 > (with-handlers ([exn? (lambda (exn) (display (exn-message exn)))])
     (error "First part" "second part"))
 First part "second part"

What you see now is that "First part" is the text of a message, while
"second part" is printed as a Scheme value. Typically, the second and
later arguments are Scheme values you want to talk about in an error
message. Here's another example:

 > (with-handlers ([exn? (lambda (exn) (display (exn-message exn)))])
     (error "This is not a string: " #\x))
 This is not a string:  #\x

At Mon, 29 Jan 2007 23:08:55 -0300, Rainer Gross wrote:
> Since I am interested in raising error messages for the end user I  
> don't have any symbol to inform. Therefore the (error src-symbol  
> format-string v ···) definition is not useful in my case. But yes I  
> want to have the format string like behavior of this function or even  
> better the java like "hello" + a + "world".

Then your entire error message should be the first argument to `error',
since you don't want to report any Scheme values in the error message.

You may want to use `string-append' or `format' to construct the error
message. Also, you probably want to call `raise-user-error' instead of
just `error', as that will avoid other potential programmerese in the
error message (such as a stack trace).

> In PLT MzScheme Language Manual section 6.2 Errors it is the second  
> form I am using:
> 
> It says:
> (error msg-string v ···) creates a message string by concatenating  
> msg-string with string versions of the vs (as produced by the current  
> error value conversion handler; see section 7.9.1.7). A space is  
> inserted before each v.

This is correct. The error value conversion handler converts a value to
a string that can be displayed to show the value's printed form:

 > ((error-value->string-handler) "hi" 10)
 "\"hi\""
 > (display ((error-value->string-handler) "hi" 10))
 "hi"

That's the right sort of conversion for showing Scheme values in an
error message, but not the right thing for inserting a string into an
end-user error message.

> I would like something like string-append but that is aware of type  
> and inserts the extra blanks as offered by (error msg-string v ...)  

We have no such function. I think it may be rare that concatenating
strings with spaces is earlier than using `format' and produces an
error message that looks right to end users (which is not to say that
it isn't the right thing for your case, only that it seems rare enough
that we don't have a built-in function for that).

Matthew