function to be memoised with argument that is function, which might be passed another memoised function

[bluaze]

a function to be memoised, say f, has an argument fn that accepts function. If such argument fn is passed another memoised function, say gm <- memoise(g), the memoised f's cache may not hit as the hash in the memoised f would change when gm is executed with new arguments and new key-value pairs are stored to gm's cache and the hash digest not only the gm itself but also its enclosing environment, which contains the caching environment

here is an example

f <- function(x) x
g <- function(fn) {i <<- fn(i) + 1; i}
i <- 0

fm <- memoise(f)
gm <- memoise(g)

gm(fm)  # first time, would be 1
gm(fm)  # second time, depends on how fm is treated in hashing, preferably still be 1

to avoid hash the entire gm, the original g could be used instead, and to avoid g's own enclosing environment problem and srcref problem, as can be seen in #84, as.character(body(g)) might be more appropriate

[codecov-io]

Codecov Report

Merging #86 into master will decrease coverage by 17.46%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #86       +/-   ##
==========================================
- Coverage   77.37%   59.9%   -17.47%     
==========================================
  Files           5       5               
  Lines         221     222        +1     
==========================================
- Hits          171     133       -38     
- Misses         50      89       +39

Impacted Files	Coverage Δ
R/memoise.R	100% <100%> (ø)	⬆️
R/cache_s3.R	0% <0%> (-100%)	⬇️

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