Created
August 29, 2021 12:35
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module Reader exposing | |
( return | |
, ask | |
, runReader | |
, local | |
, fmap | |
, chain | |
) | |
type Reader r a = Reader (r -> a) | |
id x = x | |
constant a = \_ -> a | |
return a = Reader (constant a) | |
ask = Reader id | |
runReader e reader = | |
case reader of | |
Reader r -> r(e) | |
local f reader = | |
Reader (\c -> runReader f(c) reader) | |
fmap f reader = | |
Reader (\c -> f(runReader c reader)) | |
chain f reader = | |
Reader (\c -> runReader c (f(runReader c reader))) | |
-- fun to derive | |
map f reader = | |
chain (\a -> pointed (f(a))) reader |
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