Some functions wrongly marked as pure

[vsimko]

corrplot/R/corrplot.R

Lines 1047 to 1067 in 430e501

	#' @note pure function
	#' @noRd
	draw_method_square = function(coords, values, asp_rescale_factor, fg, bg) {
	symbols(coords, add = TRUE, inches = FALSE,
	squares = asp_rescale_factor * abs(values) ^ 0.5,
	bg = bg, fg = fg)
	}
	#' @note pure function
	#' @noRd
	draw_method_color = function(coords, fg, bg) {
	symbols(coords, squares = rep(1, nrow(coords)), fg = fg, bg = bg,
	add = TRUE, inches = FALSE)
	}
	#' @note pure function
	#' @noRd
	draw_grid = function(coords, fg) {
	symbols(coords, add = TRUE, inches = FALSE, fg = fg, bg = NA,
	rectangles = matrix(1, nrow = nrow(coords), ncol = 2))
	}

If these functions are drawing something, this means they cannot be pure because they have a side effect.

[taiyun]

What is pure function?

[vsimko]

Well, a pure function is such a function which does not have any side effects and for any input it always returns the same output. For example, a mathematical function such as f(x) = x^2 is pure. However, function f(x) = sample(1:10, x) isn't pure because it returns different random values for the same input. Also a function f(x) = print(x) isn't pure because it causes a side effect (printing to console).

Pure functions behave nicely, they can be easily tested and reason about.