add more instructions for generateprecompiles.jl

deps/generateprecompiles.jl

@@ -1,6 +1,29 @@
 # To figure out what should be precompiled, run this script, then move
 # precompile_Plots.jl in precompiles_path (see below) to src/precompile.jl
 
+# This script works by using SnoopCompile to log compilations that take place
+# while running the examples on the GR backend. So SnoopCompile must be
+# installed, and StatsPlots, RDatasets, and FileIO are also required for
+# certain examples.
+
+# If precompilation fails with an UndefVarError for a module, probably what is
+# happening is that the module appears in the precompile statements, but is
+# only visible to one of Plots' dependencies, and not Plots itself. Adding the
+# module to the blacklist below will remove these precompile statements.
+
+# Anonymous functions may appear in precompile statements as functions with
+# hashes in their name. Those of the form "#something##kw" have to do with
+# compiling functions with keyword arguments, and are named reproducibly, so
+# can be kept. Others generally will not work. Currently, SnoopCompile includes
+# some anonymous functions that not reproducible, but SnoopCompile PR #30
+# (which looks about to be merged) will ensure that anonymous functions are
+# actually defined before attempting to precompile them. Alternatively, we can
+# keep only the keyword argument related anonymous functions by changing the
+# regex that SnoopCompile uses to detect anonymous functions to
+# r"#{1,2}[^\"#]+#{1,2}\d+" (see anonrex in SnoopCompile.jl). To exclude all
+# precompile statements involving anonymous functions, "#" can also be added to
+# the blacklist below.
+
 using SnoopCompile
 
 log_path = joinpath(tempdir(), "compiles.log")