add documentation for custom formats (#658)

* add vignette on managing custom-formats

* tweak vignette

* tweak news-item

* Minor edits to new vignette

Co-authored-by: Julia Silge <[email protected]>

NEWS.md

@@ -1,5 +1,7 @@
 # pins (development version)
 
+* Added vignette describing how to manage custom formats (#631, @ijlyttle).
+
 * Improved error message for `pin_versions()` (#657).
 
 # pins 1.0.3

vignettes/managing-custom-formats.Rmd

@@ -0,0 +1,95 @@
+---
+title: "Managing custom formats"
+output: rmarkdown::html_vignette
+vignette: >
+  %\VignetteIndexEntry{Managing custom formats}
+  %\VignetteEngine{knitr::rmarkdown}
+  %\VignetteEncoding{UTF-8}
+---
+
+```{r, include = FALSE}
+knitr::opts_chunk$set(
+  collapse = TRUE,
+  comment = "#>",
+  eval = rlang::is_installed("arrow")
+)
+```
+
+The pins package provides a robust set of functions to read and write standard types of files using standard tools, e.g. CSV files using `read.csv()` and `write.csv()`.
+However, from time to time, you may wish read or write using other tools. You may want to read and write:
+
+ - CSV files using readr or vroom
+ - Arrow files without using compression
+ - Parquet files
+
+An escape hatch for a customized approach is provided: `pin_upload()` and `pin_download()`. 
+The goal of this vignette is to show how you can incorporate these into your workflow.
+
+Two points to keep in mind:
+
+ - `pin_upload()` takes a vector of `paths` to local files.
+ - `pin_download()` returns a vector of `paths` to local files.
+
+We'll follow an example where we write and read uncompressed Arrow files, starting by creating a temporary board:
+
+```{r setup}
+library(pins)
+
+board <- board_temp()
+```
+
+## Upload a one-off file
+
+If you are writing a one-off file, you can do everything directly:
+
+```{r}
+pin_name <- "mtcars-arrow"
+
+# file name will be `mtcars-arrow.arrow`
+path <- fs::path_temp(fs::path_ext_set(pin_name, "arrow"))
+
+arrow::write_feather(mtcars, path, compression = "uncompressed")
+
+pin_upload(board, paths = path, name = pin_name)
+```
+
+Reading from the downloaded pin is straightforward; `pin_download()` returns a local path that can be piped to `arrow::read_feather()`:
+
+```{r}
+mtcars_download <- 
+  pin_download(board, pin_name) %>%
+  arrow::read_feather()
+
+head(mtcars_download)
+```
+
+## Function to manage uploading
+
+If you want to write more than one custom file of a certain type, or using a certain tool, you might consider writing a helper function:
+
+```{r}
+pin_upload_arrow <- function(board, x, name, ...) {
+  # path deleted when `pin_upload_arrow()` exits
+  path <- fs::path_temp(fs::path_ext_set(name, "arrow"))
+  withr::defer(fs::file_delete(path))
+ 
+  # custom writer
+  arrow::write_feather(x, path, compression = "uncompressed")
+
+  pin_upload(board, paths = path, name = name, ...) 
+}
+```
+
+This helper function is designed to work like `pin_write()`:
+
+```{r}
+pin_upload_arrow(board, x = mtcars, name = "mtcars-arrow2")
+```
+
+As before, you can pipe the result of `pin_download()` to your reader function:
+
+```{r}
+pin_download(board, name = "mtcars-arrow2") %>%
+  arrow::read_feather() %>%
+  head()
+```