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Fix toEnum for (Maybe a) #20

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Nov 10, 2016
Merged

Fix toEnum for (Maybe a) #20

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9c6a643
Fix toEnum for (Maybe a)
jacereda Nov 10, 2016
6c8fb73
Simplify
jacereda Nov 10, 2016
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4 changes: 2 additions & 2 deletions src/Data/Enum.purs
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Original file line number Diff line number Diff line change
Expand Up @@ -196,8 +196,8 @@ instance boundedEnumMaybe :: BoundedEnum a => BoundedEnum (Maybe a) where
toEnum = to cardinality
where
to :: Cardinality a -> Int -> Maybe (Maybe a)
to _ 0 = Nothing
to (Cardinality ca) n | n <= ca = Just $ toEnum (n - 1)
to _ 0 = Just Nothing
to (Cardinality ca) n | n > 0 && n <= ca = Just $ toEnum (n - 1)
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@paf31 paf31 Nov 10, 2016 •
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Why are we even doing a check here? Why not just let the call to toEnum fail?

toEnum 0 = pure Nothing
toEnum n = Just <$> toEnum (n - 1)

to _ _ = Nothing
fromEnum Nothing = 0
fromEnum (Just e) = fromEnum e + 1
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9 changes: 8 additions & 1 deletion test/Test/Data/Enum.purs
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Expand Up @@ -7,7 +7,7 @@ import Control.Monad.Eff.Console (CONSOLE, log)

import Data.Enum (class Enum, class BoundedEnum, defaultToEnum, defaultFromEnum,
defaultCardinality, enumFromTo, enumFromThenTo, upFrom,
downFrom)
downFrom, toEnum)
import Data.Maybe (Maybe(..))

import Test.Assert (ASSERT, assert)
Expand Down Expand Up @@ -63,3 +63,10 @@ testEnum = do
assert $ downFrom D == [C, B, A]
assert $ downFrom B == [ A]
assert $ downFrom A == [ ]

log "BoundedEnum (Maybe Boolean)"
assert $ toEnum (-1) == Nothing :: Maybe (Maybe Boolean)
assert $ toEnum 0 == Just Nothing :: Maybe (Maybe Boolean)
assert $ toEnum 1 == Just (Just false) :: Maybe (Maybe Boolean)
assert $ toEnum 2 == Just (Just true) :: Maybe (Maybe Boolean)
assert $ toEnum 3 == Nothing :: Maybe (Maybe Boolean)