add vignette on introducing styler

vignettes/introducing_styler.Rmd

@@ -0,0 +1,178 @@
+---
+title: "An introduction to styler"
+author: "Lorenz Walthert"
+date: "`r Sys.Date()`"
+output: rmarkdown::html_vignette
+vignette: >
+  %\VignetteIndexEntry{An introduction to styler}
+  %\VignetteEngine{knitr::rmarkdown}
+  %\VignetteEncoding{UTF-8}
+---
+
+This vignette introduces the basic functionality of styler and showcases
+how styler applies a few rules of the 
+[tidyverse style guide](http://style.tidyverse.org/index.html) to example code.
+Note that you can create your own style guide and customize styler even further, 
+as described in the vignette "Customizing styler".
+
+```{r, echo = FALSE}
+knitr::opts_chunk$set(echo = TRUE, comment = "")
+knitr::knit_engines$set(list(
+  styler = function(options) {
+    options$comment <- ""
+    knitr::engine_output(
+      options,
+
+      c("# Before", options$code),
+      c("# After", styler::style_text(options$code))
+    )
+  }
+))
+
+```
+
+It's possible to use different levels of 'invasiveness', as described in the 
+help file for the only style guide implemented so far, which is the 
+[tidyverse style guide](http://style.tidyverse.org/index.html). The style guide 
+in use is passed to the styling function (i.e `style_text()` and friends) via 
+the `style` argument, which defaults to `tidyverse_style`. In addition to this 
+argument, there are further customization options. For example, we can limit 
+ourselves to styling just spacing information by indicating this with the 
+`scope` argument:
+
+```{r}
+library("styler")
+library("magrittr")
+style_text("a=3; 2", scope = "spaces")
+```
+
+Or, on the other extreme of the scale, styling spaces, indention, line breaks 
+and tokens (which is the default):
+
+```{r}
+style_text("a=3; 2", scope = "tokens")
+```
+
+
+`scope` always includes less-invasive styling than the option chosen, 
+e.g. `spaces = "line_breaks"` includes styling spaces and indention in addition 
+to line breaks.
+
+
+We can also choose to style line breaks but not tokens:
+```{r}
+style_text("if(x) {66 } else {a=3}", scope = "line_breaks")
+```
+
+Note that `scope = "spaces"` does not touch indention
+```{r}
+code <- c(
+    "a <- function() { ", 
+    "                a=3", 
+    "}"
+)
+
+style_text(code, scope = "spaces")
+```
+
+But `scope = "indention"` - as the name says - does.
+```{r}
+style_text(code, scope = "indention")
+```
+
+
+Another option that is helpful to determine the level of 'invasiveness' is
+`strict`. If set to `TRUE`, spaces and line breaks before or after tokens are
+set to either zero or one. However, in some situations this might be undesirable,
+as the following example shows:
+
+```{r}
+style_text(
+  "data_frame(
+     small  = 2 ,
+     medium = 4,#comment without space
+     large  = 6
+   )", strict = FALSE
+)
+```
+
+We prefer to keep the equal sign after "small", "medium" and large aligned,
+so we set `strict = FALSE` to set spacing to *at least* one around `=`.
+
+Also, spaces before comments are preserved with that option.
+
+```{r}
+style_text(
+  "a <-   'one'   #just one
+   abc <- 'three' # three", 
+  strict = FALSE
+)
+```
+
+
+Though simple, hopefully the above examples convey some of the flexibility of 
+the configuration options available in `styler`. Let us for now focus on a 
+configuration with `strict = TRUE` and `scope = "tokens"` and illustrate a few 
+more examples of code before and after styling.
+
+`styler` can identify and handle unary operators and other math tokens:
+
+```{styler}
+1++1-1-1/2
+```
+
+It can also format complicated expressions that involve line breaking and 
+indention based on both brace expressions and operators:
+
+```{styler}
+if (x >3) {stop("this is an error")} else {
+c(there_are_fairly_long,
+1 / 33 * 
+2 * long_long_variable_names)%>% k(
+
+) }
+```
+
+Lines are broken after `(` if a function call spans multiple lines:
+
+```{styler}
+do_a_long_and_complicated_fun_cal("which", has, way, to, 
+                              "and longer then lorem ipsum in its full length"
+                              )
+```
+
+`styler` replaces `=` with `<-` for assignment:
+```{styler}
+one = "one string"
+```
+
+It converts single quotes within strings if necessary:
+```{styler}
+one <- 'one string'
+two <- "one string in a 'string'"
+```
+
+And adds braces to function calls in pipes:
+
+```{styler}
+a %>%
+  b %>%
+  c
+```
+
+Function declarations are indented if multi-line:
+
+```{styler}
+my_fun <- function(x, 
+y, 
+z) {
+  just(z)
+}
+```
+
+`styler` can also deal with tidyeval syntax:
+
+```{styler}
+mtcars %>%
+  group_by( !!my_vars )
+```