Re: Determine if a parameter is a function

*To*: mathgroup at smc.vnet.net*Subject*: [mg101754] Re: Determine if a parameter is a function*From*: Szabolcs <szhorvat at gmail.com>*Date*: Thu, 16 Jul 2009 08:18:22 -0400 (EDT)*References*: <h3kddu$g35$1@smc.vnet.net>

On Jul 15, 2:09 pm, Peter Breitfeld <phbrf at t-online.de> wrote: > Suppose I have a function eg > > myfunc[f_,x_]:= <some definitions> > > f should be a pure function like (#^2&) or Function[{x},x^2] or a named > function either self defined, like > > f[x_]:=x^2 or g[x_]=x^2 > > or built-in like Sin, Log, ... > > How can I test if f is any of these, to be able to yield a message on > wrong input? > > I found that the pure-functions have Head Function, but all the others > have Head Symbol, so asking for the head is not sufficient. > I don't believe that it is possible to do this in a reliable way, so my suggestion is that you don't test this at all. Trying to test it might cause more harm than good. A "function" defined like f[x_] := x^2 will have DownValues (check DownValues[f]). But now consider something like g[n_][x_] := x^n. I, as a user, would expect that I can pass e.g. g[2] to your function, but g[2] is neither a Function, nor a Symbol ... furthermore, g doesn't even have DownValues (only SubValues). Built-in functions don't even have DownValues (or even worse: will only have them after they have been used for the first time in a session). Also, the fact that a symbol has DownValues is no guarantee that it behaves like a "proper" function.. Szabolcs P.S. If myfunc[] takes functions in the mathematical sense (rather than in a programming sense), sometimes it is advantageous to use a syntax similar to myfunc[1+x^2+x^3, x] rather than myfunc[ 1+#^2+#^3 & ]. However, whether this is an advantage or disadvantage depends on what myfunc[] actually is/does.

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**Re: Determine if a parameter is a function**

**Re: Determine if a parameter is a function**