System.Args.Optional

Type-classes to model optional function arguments. Use with care.

Lots of helpful type-inference is lost in using this technique.

Documentation

class Optional1 a b x where

One optional argument.

>>> let timesTwo :: Optional1 Integer Integer x => x; timesTwo = optional1 (*2) 7 in timesTwo :: Integer
14

>>> let timesTwo :: Optional1 Integer Integer x => x; timesTwo = optional1 (*2) 7 in timesTwo (12 :: Integer) :: Integer
24

>>> let reverseI :: Optional1 [Integer] [Integer] x => x; reverseI = optional1 reverse ([] :: [Integer]) in reverseI ([1,2,3] :: [Integer]) :: [Integer]
[3,2,1]

>>> let reverseI :: Optional1 [Integer] [Integer] x => x; reverseI = optional1 reverse ([] :: [Integer]) in reverseI :: [Integer]
[]

>>> let reverseI :: Optional1 [Integer] [Integer] x => x; reverseI = optional1 reverse [99, 98, 97] in reverseI :: [Integer]
[97,98,99]

>>> let filterI :: Optional1 (Integer -> Bool) [Integer] x => x; filterI = optional1 (\p -> filter p [1..20]) (even :: Integer -> Bool) in filterI :: [Integer]
[2,4,6,8,10,12,14,16,18,20]

>>> let filterI :: Optional1 (Integer -> Bool) [Integer] x => x; filterI = optional1 (\p -> filter p [1..20]) (even :: Integer -> Bool) in filterI (odd :: Integer -> Bool) :: [Integer]
[1,3,5,7,9,11,13,15,17,19]

Methods

optional1 :: (a -> b) -> a -> x

Instances

Optional1 a b b

Optional1 a b (a -> b)

class Optional2 a b c x where

Two optional arguments, in the order of a then b.

>>> let append :: Optional2 String String String x => x; append = optional2 (++) "abc" "def" in append :: String
"abcdef"

>>> let append :: Optional2 String String String x => x; append = optional2 (++) "abc" "def" in append "xyz" :: String
"xyzdef"

>>> let append :: Optional2 String String String x => x; append = optional2 (++) "abc" "def" in append "uvw" :: String
"uvwdef"

>>> let append :: Optional2 String String String x => x; append = optional2 (++) "abc" "def" in append "uvw" "xyz" :: String
"uvwxyz"

>>> let filterS :: Optional2 (Char -> Bool) String String x => x; filterS = optional2 filter isUpper "AbCdEfGhI" in filterS :: String
"ACEGI"

>>> let filterS :: Optional2 (Char -> Bool) String String x => x; filterS = optional2 filter isUpper "AbCdEfGhI" in filterS isLower :: String
"bdfh"

>>> let filterS :: Optional2 (Char -> Bool) String String x => x; filterS = optional2 filter isUpper "AbCdEfGhI" in filterS isLower "tUvWxYz" :: String
"tvxz"

Methods

optional2 :: (a -> b -> c) -> a -> b -> x

Instances

Optional2 a b c c

Optional1 b c x => Optional2 a b c (a -> x)