Weird feature when creating function lists with apply

Uffe H?gsbro 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
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