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klmr/list_comprehension.r

Created February 12, 2016 15:17
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Haskell-like list comprehension for R
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list_comprehension.r
# Dummy object, only required for name resolution.
set = structure(list(), class = 'set')
print.set = function (x, ...) invisible(x)
`[.set` = function (set, expr, filter) {
expr = substitute(expr)
filter = substitute(filter)
stopifnot(identical(expr[[1]], quote(`<-`)))
stopifnot(identical(expr[[2]][[1]], quote(`|`)))
map = expr[[2]][[2]]
gen = expr[[3]]
var = expr[[2]][[3]]
stopifnot(length(var) == 1 && is.name(var))
var = deparse(var, backtick = TRUE)
range = eval.parent(gen)
closure = function (formals, body, envir)
eval(call('function', as.pairlist(formals), body), envir)
formals = as.pairlist(setNames(c(quote(expr = )), var))
f = closure(formals, filter, parent.frame())
g = closure(formals, map, parent.frame())
sapply(Filter(f, range), g)
}
# Example:
set[x + 1 | x <- 1 : 10, x %% 2 == 0]
# [1] 3 5 7 9 11
# Implementing multi-variable list comprehension left as exercise for the reader.
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