[Rd] (inappropriate) manipulation of expression objects causes segfault (PR#7326)

[Peter Dalgaard]

deepayan at stat.wisc.edu writes:

> > foo <- expression(alpha, beta, gamma)
> > foo[2]
> expression(beta)
> > foo[2] <- NA
> > foo ## or str(foo)
> Segmentation fault
> 
> (Same behaviour in R 1.9.1)
> 
> 'foo[[2]] <- NA' works fine, though.

Right. The NA is not important; foo[2] <- 1 crashes R just as
effectively. We should likely either give an error or demote [ to [[.
The latter is what happens with lists, and expressions are basically
just list of call objects (or names or constants). To wit:

> x <- list(1,2,3)
> x[2] <- 1
> x
[[1]]
[1] 1

[[2]]
[1] 1

[[3]]
[1] 3

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