[plt-scheme] possible typo in reference?

On Oct 6, 2009, at 6:07 PM, Todd O'Bryan wrote:

> I'm reading through the Evaluation Model of the Reference and think I
> found a typo...
>
> ----
> Section 1.1.2 Tail Position
>
> For example, the (+ 1 1) expression is not in tail position with
> respect to (- 4 (+ 1 1)). To illustrate, we use the notation C[expr]
> to mean the expression that is produced by substituting expr in place
> of [] in the continuation C:
>
>  C[(- 4 (+ 1 1))] → C[(- 4 2)]
>
> In this case, the continuation for reducing (+ 1 1) is C[(+ 4 [])],  
> not just C.
> ----
>
> Should the last sentence say, "In this case, the continuation for
> reducing (+ 1 1) is C[(+ 4 [])], not just C[(+ 1 1)]." or am I still
> not understanding this?

The text is correct as written (though there seem to be some pluses  
and minuses changing back and forth).

A continuation (or context) here is something with a hole.  If you  
plug something without a hole into the hole (as in C[(+ 1 1)]), then  
that is no longer a context; it's just a plain old expression. If you  
plug something with a hole in it into the hole (as in C[(+ 4 [])], you  
get another context.  So C is a context, and C[(+ 4 [])] is a context,  
but C[(+ 1 1)] is not a context.

For my word "context," I'm guessing that you'll want to use  
"continuation."

John





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