Options for caching / memoization / hashing in R

I am trying to find a simple way to use something like Perl's hash functions in R (essentially caching), as I intended to do both Perl-style hashing and write my own memoisation of calculations. However, others have beaten me to the punch and have packages for memoisation. The more I dig, the more I find, e.g.memoise and R.cache, but differences aren't readily clear. In addition, it's not clear how else one can get Perl-style hashes (or Python-style dictionaries) and write one's own memoization, other than to use the hash package, which doesn't seem to underpin the two memoization packages.

Since I can find no information on CRAN or elsewhere to distinguish between the options, perhaps this should be a community wiki question on SO: What are the options for memoization and caching in R, and what are their differences?

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As a basis for comparison, here is a list of the options I've found. Also, it seems to me that all depend on hashing, so I'll note the hashing options as well. Key/value storage is somewhat related, but opens a huge can of worms regarding DB systems (e.g. BerkeleyDB, Redis, MemcacheDB and scores of others).

It looks like the options are:

Hashing

• digest - provides hashing for arbitrary R objects.

Memoization

• memoise - a very simple tool for memoization of functions.
• R.cache - offers more functionality for memoization, though it seems some of the functions lack examples.

Caching

• hash - Provides caching functionality akin to Perl's hashes and Python dictionaries.

Key/value storage

These are basic options for external storage of R objects.

• stashr
• filehash

Checkpointing

• cacher - this seems to be more akin to checkpointing.
• CodeDepends - An OmegaHat project that underpins cacher and provides some useful functionality.
• DMTCP (not an R package) - appears to support checkpointing in a bunch of languages, and a developer recently sought assistance testing DMTCP checkpointing in R.

Other

• Base R supports: named vectors and lists, row and column names of data frames, and names of items in environments. It seems to me that using a list is a bit of a kludge. (There's also pairlist, but it is deprecated.)
• The data.table package supports rapid lookups of elements in a data table.

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Use case

Although I'm mostly interested in knowing the options, I have two basic use cases that arise:

1. Caching: Simple counting of strings. [Note: This isn't for NLP, but general use, so NLP libraries are overkill; tables are inadequate because I prefer not to wait until the entire set of strings are loaded into memory. Perl-style hashes are at the right level of utility.]
2. Memoization of monstrous calculations.

These really arise because I'm digging in to the profiling of some slooooow code and I'd really like to just count simple strings and see if I can speed up some calculations via memoization. Being able to hash the input values, even if I don't memoize, would let me see if memoization can help.

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Note 1: The CRAN Task View on Reproducible Research lists a couple of the packages (cacher and R.cache), but there is no elaboration on usage options.

Note 2: To aid others looking for related code, here a few notes on some of the authors or packages. Some of the authors use SO. :)

• Dirk Eddelbuettel: digest - a lot of other packages depend on this.
• Roger Peng: cacher, filehash, stashR - these address different problems in different ways; see Roger's site for more packages.
• Christopher Brown: hash - Seems to be a useful package, but the links to ODG are down, unfortunately.
• Henrik Bengtsson: R.cache & Hadley Wickham: memoise -- it's not yet clear when to prefer one package over the other.

Note 3: Some people use memoise/memoisation others use memoize/memoization. Just a note if you're searching around. Henrik uses "z" and Hadley uses "s".

I did not have luck with memoise because it gave a 'too deep recursive' problem to some functions of a package I tried it with. With R.cache I had better luck. Following is more annotated code I adapted from the R.cache documentation. The code shows different options for doing caching:

# Workaround to avoid question when loading R.cache library
dir.create(path="~/.Rcache", showWarnings=F) 
library("R.cache")
setCacheRootPath(path="./.Rcache") # Create .Rcache at current working dir
# In case we need the cache path, but not used in this example.
cache.root = getCacheRootPath() 
simulate <- function(mean, sd) {
    # 1. Try to load cached data, if already generated
    key <- list(mean, sd)
    data <- loadCache(key)
    if (!is.null(data)) {
        cat("Loaded cached data\n")
        return(data);
    }
    # 2. If not available, generate it.
    cat("Generating data from scratch...")
    data <- rnorm(1000, mean=mean, sd=sd)
    Sys.sleep(1) # Emulate slow algorithm
    cat("ok\n")
    saveCache(data, key=key, comment="simulate()")
    data;
}
data <- simulate(2.3, 3.0)
data <- simulate(2.3, 3.5)
a = 2.3
b = 3.0
data <- simulate(a, b) # Will load cached data, params are checked by value
# Clean up
file.remove(findCache(key=list(2.3,3.0)))
file.remove(findCache(key=list(2.3,3.5)))

simulate2 <- function(mean, sd) {
    data <- rnorm(1000, mean=mean, sd=sd)
    Sys.sleep(1) # Emulate slow algorithm
    cat("Done generating data from scratch\n")
    data;
}
# Easy step to memoize a function
# aslo possible to resassign function name.
This would work with any functions from external packages. 
mzs <- addMemoization(simulate2)

data <- mzs(2.3, 3.0)
data <- mzs(2.3, 3.5)
data <- mzs(2.3, 3.0) # Will load cached data
# aslo possible to resassign function name.
# but different memoizations of the same 
# function will return the same cache result
# if input params are the same
simulate2 <- addMemoization(simulate2)
data <- simulate2(2.3, 3.0)

# If the expression being evaluated depends on
# "input" objects, then these must be be specified
# explicitly as "key" objects.
for (ii in 1:2) {
    for (kk in 1:3) {
        cat(sprintf("Iteration #%d:\n", kk))
        res <- evalWithMemoization({
            cat("Evaluating expression...")
            a <- kk
            Sys.sleep(1)
            cat("done\n")
            a
        }, key=list(kk=kk))
        # expressions inside 'res' are skipped on the repeated run
        print(res)
        # Sanity checks
        stopifnot(a == kk)
        # Clean up
        rm(a)
    } # for (kk ...)
} # for (ii ...)

For simple counting of strings (and not using table or similar), a multiset data structure seems like a good fit. The environment object can be used to emulate this.

# Define the insert function for a multiset
msetInsert <- function(mset, s) {
	if (exists(s, mset, inherits=FALSE)) {
		mset[[s]] <- mset[[s]] + 1L
	} else {
		mset[[s]] <- 1L	
	}
}

# First we generate a bunch of strings
n <- 1e5L  # Total number of strings
nus <- 1e3L  # Number of unique strings
ustrs <- paste("Str", seq_len(nus))

set.seed(42)
strs <- sample(ustrs, n, replace=TRUE)


# Now we use an environment as our multiset    
mset <- new.env(TRUE, emptyenv()) # Ensure hashing is enabled

# ...and insert the strings one by one...
for (s in strs) {
    msetInsert(mset, s)
}

# Now we should have nus unique strings in the multiset    
identical(nus, length(mset))

# And the names should be correct
identical(sort(ustrs), sort(names(as.list(mset))))

# ...And an example of getting the count for a specific string
mset[["Str 3"]] # "Str 3" instance count (97)

Related to @biocyperman solution. R.cache has a wrapping function for avoiding the loading, saving and evaluation of the cache. See the modified function:

R.cache provide a wrapper for loading, evaluating, saving. You can simplify your code like that:

simulate <- function(mean, sd) {
key <- list(mean, sd)
data <- evalWithMemoization(key = key, expr = {
    cat("Generating data from scratch...")
    data <- rnorm(1000, mean=mean, sd=sd)
    Sys.sleep(1) # Emulate slow algorithm
    cat("ok\n")
    data})
}