[R] Weird feature when creating function lists with apply

[Peter Dalgaard BSA]

Uffe Høgsbro Thygesen <uht at dfu.min.dk> writes:

> Thanks, Peter and Prof. Ripley, for your helpful replies.
> 
> Referring to Prof. Ripley's reply, when I do
> 
> f3 <- function(a) substitute(function(x) a*x, list(a = a))
> f <- lapply(1:4, f3)[[1]]
> 
> then f is not a function:
> 
> > is.function(f)
> [1] FALSE
> 
> but 
> 
> > is.function(eval(f))
> [1] TRUE
...
> I do still not understand what f is, then, except something that evaluates
> to a function. My conclusion is that I need to start my head, too, and
> understand this lazy evaluation.

This part is not about lazy evaluation. This is because substitute
returns an object of mode "call", sometimes called an "unevaluated
expression", (which is a bit unfortunate because there is also an
"expression" mode, of which it is not...)

You could try picking it apart:
> f[[1]]
function
> f[[2]]
$x


> f[[3]]
1 * x
> f[[3]][[1]]
*
[..etc..]

> g <- eval(f)
> g
function(x) a*x

!!! Now *that* is truly weird.... The "source" atribute for g is
getting set, seemingly out of nowhere, to something that should not be
part of f. (This is a bug, but some nasty trickery is involved in
these matters and if might be difficult to fix.)

However if we clear
> attributes(g)<-NULL
> g
function (x) 
1 * x

you can see that g is the actual function, not the call to the
function creator.


-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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