[ruby/csv] Recipes for field converters (#177)

https://github.com/ruby/csv/commit/aea896f030

doc/csv/recipes.rdoc

@@ -1,5 +1,8 @@
 == Recipes
 
+All code snippets on this page assume that the following has been executed:
+  require 'csv'
+
 === Contents
 
 - {Parsing: Source Formats}[#label-Parsing-3A+Source+Formats]
@@ -12,6 +15,10 @@
   - {Parse from IO Stream}[#label-Parse+from+IO+Stream]
     - {Parse from IO Stream Without Headers}[#label-Parse+from+IO+Stream+Without+Headers]
     - {Parse from IO Stream with Headers}[#label-Parse+from+IO+Stream+with+Headers]
+- {Parsing: Field Converters}[#label-Parsing-3A+Field+Converters]
+  - {Convert Fields to Objects}[#label-Convert+Fields+to+Objects]
+    - {Convert Fields to Objects Using Built-In Converters}[#label-Convert+Fields+to+Objects+Using+Built-In+Converters]
+    - {Convert Fields to Objects Using Custom Converters}[#label-Convert+Fields+to+Objects+Using+Custom+Converters]
 - {Generating: Output Formats}[#label-Generating-3A+Output+Formats]
   - {Generate to String}[#label-Generate+to+String]
     - {Generate to String Without Headers}[#label-Generate+to+String+Without+Headers]
@@ -152,6 +159,52 @@ Output:
   #<CSV::Row "Name":"bar" "Value":"1">
   #<CSV::Row "Name":"baz" "Value":"2">
 
+=== Parsing: Field Converters
+
+==== Convert Fields to Objects
+
+Use field converters to change parsed Strings into other, more specific, object.
+
+==== Convert Fields to Objects Using Built-In Converters
+
+Without converters (all fields parsed as Strings):
+  source = "0,1.1,2020-09-19"
+  parsed = CSV.parse(source)
+  parsed # => [["0", "1.1", "2020-09-19"]]
+  parsed.first.each {|field| p field.class }
+Output:
+  String
+  String
+  String
+
+With built-in converters (see {Built-In Field Converters}[../../CSV.html#class-CSV-label-Built-In+Field+Converters]):
+  parsed = CSV.parse(source, converters: :all)
+  parsed # => [[0, 1.1, #<DateTime: 2020-09-19T00:00:00+00:00 ((2459112j,0s,0n),+0s,2299161j)>]]
+  parsed.first.each {|field| p field.class }
+Output:
+  Integer
+  Float
+  DateTime
+
+==== Convert Fields to Objects Using Custom Converters
+
+Define a custom field converter:
+  strip_converter = proc {|field| field.strip }
+
+Without the new converter:
+  string = " foo , 0 \n bar , 1 \n baz , 2 \n"
+  array = CSV.parse(string)
+  array # => [[" foo ", " 0 "], [" bar ", " 1 "], [" baz ", " 2 "]]
+
+With the new converter:
+  array = CSV.parse(string, converters: strip_converter)
+  array # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]
+
+You can also register a custom field converter, then refer to it by name:
+  CSV::Converters[:strip] = strip_converter
+  array = CSV.parse(string, converters: :strip)
+  array # => [["foo", "0"], ["bar", "1"], ["baz", "2"]]
+
 === Generating: Output Formats
 
 ==== Generate to \String Without Headers