New script - illustrating the pipe

RPR-Pipe.R

@@ -0,0 +1,135 @@
+# tocID <- "RPR-Pipe.R"
+#
+# Purpose:  A Bioinformatics Course:
+#              Discussing pipe operators.
+#
+# Version:  1.0
+#
+# Date:     2021  10
+# Author:   Boris Steipe (boris.steipe@utoronto.ca)
+#
+# Versions:
+#           1.0    New code
+#
+#
+# TODO:
+#   - find more interesting examples
+#
+# == DO NOT SIMPLY  source()  THIS FILE! =======================================
+#
+# If there are portions you don't understand, use R's help system, Google for an
+# answer, or ask your instructor. Don't continue if you don't understand what's
+# going on. That's not how it works ...
+#
+# ==============================================================================
+
+
+#TOC> ==========================================================================
+#TOC>
+#TOC>   Section  Title                            Line
+#TOC> ------------------------------------------------
+#TOC>   1        Pipe  Concept                      41
+#TOC>   2        Nested Expression                  73
+#TOC>   3        magrittr:: Pipe                    78
+#TOC>   4        Base R Pipe                        93
+#TOC>   5        Intermediate Assignment           108
+#TOC>   6        Postscript                        127
+#TOC>
+#TOC> ==========================================================================
+
+
+# =    1  Pipe  Concept  =======================================================
+
+# Pipes are actually an awesome idea for any code that implements a workflow -
+# a sequence of operations, each of which transforms data in a specialized way.
+#
+# This principle is familiar from maths: chained functions. If have a function
+# y = f(x) and want to use those results as in z = g(y), I can just write
+# z = g(f(x))
+#
+# On the unix command line, pipes were used from the very beginning, implemented
+# with the "|" pipe character.
+#
+# In R, the magrittr package provided the %>% operator, and recently the %|>%
+# operator has been introduced into base R.
+#
+# However there are alternatives: intermediate assignment, and nested functions
+# that have always existed in bas R anyway.
+#
+# Let us look at an example. In writing this, I found out that virtually
+# ALL non-trivial examples I cvame up with don't translate well into this idiom
+# at all. It is actually quite limited to simple filtering operations on
+# data. A more intersting example might be added in the future, let me know if
+# you have a good idea.
+#
+# A somwhat contrived example is to soart a list of files by the
+# length of the file names:
+
+myFiles <- list.files(pattern = "\\.R$")
+
+# nchar() gives the number of characters in a string, order() produces indices
+# that map an array to its sorted form.
+#
+# =    2  Nested Expression  ===================================================
+
+myFiles[order(nchar(myFiles))]
+
+
+# =    3  magrittr:: Pipe  =====================================================
+
+if (! requireNamespace("magrittr", quietly = TRUE)) {
+  install.packages("magrittr")
+}
+# Package information:
+#  library(help = magrittr)       # basic information
+#  browseVignettes("magrittr")    # available vignettes
+#  data(package = "magrittr")     # available datasets
+
+
+library(magrittr)
+
+nchar(myFiles) %>% order %>% myFiles[.]
+
+# =    4  Base R Pipe  =========================================================
+
+# Since version 4.1, base R now supports a pipe operator without the need
+# to load a special package. Such an introductions of external functionality
+# into the language is very rare.
+#
+# Unfortunately it won't (yet) work with the '[' function, so we need to write
+# an intermediate fucntion for this example
+extract <- function(x, v) {
+  return(v[x])
+}
+
+nchar(myFiles) |> order() |> extract(myFiles)
+
+
+# =    5  Intermediate Assignment  =============================================
+
+# So what's the problem? As you can see, the piped code may be concise and
+# expressive. But there is also a large amount of implicit assignment and
+# processing going on and that is usually a bad idea because it makes code hard
+# to maintain. I am NOT a big fan of the nested syntax, but I don't think that
+# replacing it with the pipe makes things much better. My preferred idiom is
+# to use intermediate assignments. Only then is it convenient to examine
+# the code step by step and validate every single step. And that is the most
+# important objective at all: no code is good if it doe not compute
+# correctly.
+
+
+x <- nchar(myFiles)
+x <- order(x)
+myFiles[x]
+
+
+
+# =    6  Postscript  ==========================================================
+
+# I tried to write an example that strips all comments from a list of files, and
+# another example that finds all files that were not yet updated this year
+# (according to the "# Date: in the header). Neither examples can be well
+# written without intermediate assignments, or at least sapply() functions
+# that are not simpler at all than the intermediate assignment.
+
+# [END]