Haskell-like list comprehension for R

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.