Add early sanity check on some delayed operations

This sanity check will trigger an error in situations like:
  M <- DelayedArray(matrix(character(12), ncol=3))
  M2 <- log(M)  # error!
Before this change, the above code was working and the error was delayed
until the user would display or realize M2. The check is cheap so better
fail early than late!

DESCRIPTION

@@ -8,7 +8,7 @@ Description: Wrapping an array-like object (typically an on-disk object) in
 	also works on in-memory array-like objects like DataFrame objects
 	(typically with Rle columns), Matrix objects, and ordinary arrays and
 	data frames.
-Version: 0.7.29
+Version: 0.7.30
 Encoding: UTF-8
 Author: Hervé Pagès <hpages@fredhutch.org>, with contributions from
 	Peter Hickey <peter.hickey@gmail.com>

R/DelayedArray-class.R

@@ -422,7 +422,8 @@ stash_DelayedUnaryIsoOp <- function(x, OP, Largs=list(), Rargs=list(),
 {
     stopifnot(is(x, "DelayedArray"))
     op <- new_DelayedUnaryIsoOp(x@seed, OP=OP, Largs=Largs, Rargs=Rargs,
-                                        Lalong=Lalong, Ralong=Ralong)
+                                        Lalong=Lalong, Ralong=Ralong,
+                                        check.op=TRUE)
     DelayedArray(op)
 }
 

R/DelayedOp-class.R

@@ -405,7 +405,8 @@ setClass("DelayedUnaryIsoOp",
 
 new_DelayedUnaryIsoOp <- function(seed=new("array"),
                                   OP=identity, Largs=list(), Rargs=list(),
-                                  Lalong=NA, Ralong=NA)
+                                  Lalong=NA, Ralong=NA,
+                                  check.op=FALSE)
 {
     seed_dim <- dim(seed)
     if (length(seed_dim) == 0L)
@@ -417,8 +418,21 @@ new_DelayedUnaryIsoOp <- function(seed=new("array"),
 
     OP <- match.fun(OP)
 
-    new2("DelayedUnaryIsoOp", seed=seed, OP=OP, Largs=Largs, Rargs=Rargs,
-                              Lalong=Lalong, Ralong=Ralong)
+    ans <- new2("DelayedUnaryIsoOp", seed=seed,
+                                     OP=OP, Largs=Largs, Rargs=Rargs,
+                                     Lalong=Lalong, Ralong=Ralong)
+    if (check.op) {
+        ## We quickly test the validity of the operation by calling type()
+        ## on the returned object. This will fail if the operation cannot
+        ## be applied e.g. if the user does something like:
+        ##   M <- DelayedArray(matrix(character(12), ncol=3))
+        ##   M2 <- log(M)
+        ## The test is cheap and type() will be called anyway by show()
+        ## later when the user tries to display M2. Better fail early than
+        ## late!
+        type(ans)  # we ignore the returned value
+    }
+    ans
 }
 
 ### S3/S4 combo for summary.DelayedUnaryIsoOp