A library for declaring algebraic data types in Ruby.
gem install adt
ADT provides a DSL for specifying the cases in an algebraic data type.
require 'adt'
class ValidatedValue
extend ADT
cases do
missing
invalid(:reason)
ok(:value)
end
end
# An Enumeration (nullary constructors only)
class State
extend ADT
cases do
snafu
smoking # 'Nullary contructor' means it takes no arguments
end
end
What you now have:
- Constructors for each of the cases: Type.case(arg)
- A
foldmethod, for matching on all the cases. - A good #== and #inspect implementation
- #case? and #when__case_(handle_case_proc, default_proc) for dealing with a single case
Check the documentation for more information.
Construction:
# Create values
mine = ValidatedValue.ok(5)
missing = ValidatedValue.missing
invalid = ValidatedValue.invalid("Wrong number!")
Folding:
# Define operations on a value, only the proc matching the value's case will be
# executed
mine.fold(
proc { |value| value },
proc { "missing default" }
proc { |reason| raise "gah. Invalid is terrible" }
)
# Use an alias to #fold, named after the type:
mine.validated_value(
:ok => proc { |value| value },
:missing => proc { "missing default " },
:invalid => proc { |reason| raise "gah. Invalid is terrible!" }
)
Support methods:
mine.ok? # <= true
mine.when_missing(proc { "I'm missing!" }, proc { "It's okay I'm around" })
# == does what you expect.
mine == ValidatedValue.missing # <= false
mine == ValidatedValue.ok(5) # <= true
# <=>
ValidatedValue.ok(5) <=> ValidatedValue.ok(3) # <= 1 # Ordering is by the inner value(s), if the cases match
ValidatedValue.ok(5) <=> ValidatedValue.missing # <= 1 # Otherwise it is by increasing order in which the cases are defined
# to_a
ValidatedValue.ok(5).to_a == [5]
ValidatedValue.missing.to_a = []
# Inspect looks good.
mine.inspect # <= "#<ValidatedValue ok value:5>"
For the enumeration only:
State.all_values # <= [State.snafu, State.smoking]
State.snafu.to_i # <= 1
State.from_i(2) # <= State.smoking
Case info:
State.snafu.case_name == "snafu"
ValidatedValue.ok(3).case_arity == 1
State.snafu.case_index = 2