Introduce pseudovariadic functions.

scribblings/functions.scrbl

@@ -18,7 +18,37 @@ Boolean functions, etc.).
 		 [sandbox-memory-limit 50])
    (make-evaluator 'typed/racket #:requires '((submod "functions.rkt" typed)))))
 
-@section{Tabulating functions}
+@section{Pseudovariadic functions}
+
+Functions for @seclink["tabulating"]{tabulating functions} take as an argument
+a function to tabulate or a list of functions to tabulate.  Writing the type of
+such functions in Typed Racket and generalizing on the number of the arguments
+is hard, and using functions with such types seems even harder.
+The @seclink["tabulating"]{following section} contains some examples,
+illustrating among other things the difficulties of typing
+tabulating functions.
+
+The type of @racket[apply] does not help in this situation, because Typed
+Racket treats @racket[apply] in
+@hyperlink["https://racket.discourse.group/t/replicating-the-type-of-apply/770/3"]{a
+special way}.  This means that a user-defined function with the same type as
+@racket[apply] and directly calling it will not work in the same way.
+
+@examples[#:eval functions-evaluator
+apply
+(define myapply apply)
+myapply
+(apply (λ (x y) (and x y)) '(#t #f))
+(eval:error (myapply (λ (x y) (and x y)) '(#t #f)))
+]
+
+One way to work around this issue is to write functions which disguise as
+variadic functions of type @racket[(-> a * b)], but which throw an exception
+when they receive a number of arguments different from a given constant value.
+Such functions are called @italic{pseudovariadic functions} in
+this documentation.
+
+@section[#:tag "tabulating"]{Tabulating functions}
 
 @defproc[(tabulate [func (-> a ... b)]
 		   [doms (List (Listof a) ... a)])