StdMaybe.dcl 1.05 KB
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definition module StdMaybe

//	********************************************************************************
//	Clean Standard Object I/O library, version 1.2
//	
//	StdMaybe defines the Maybe type.
//	********************************************************************************

from	StdFunc			import St
from	StdOverloaded	import ==
from	StdIOBasic		import IdFun

::	Maybe x
	=	Just x
	|	Nothing

isJust		:: !(Maybe .x) -> Bool		// case @1 of (Just _) -> True; _ -> False
isNothing	:: !(Maybe .x) -> Bool		// not o isJust
fromJust	:: !(Maybe .x) -> .x		// \(Just x) -> x

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// for possibly unique elements:
u_isJust	:: !(Maybe .x) -> (!Bool, !Maybe .x)
u_isNothing	:: !(Maybe .x) -> (!Bool, !Maybe .x)

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accMaybe	:: .(St .x .a) !(Maybe .x) -> (!Maybe .a,!Maybe .x)
// accMaybe f (Just x) = (Just (fst (f x)),Just (snd (f x)))
// accMaybe f Nothing  = (Nothing,Nothing)

mapMaybe	:: .(.x -> .y) !(Maybe .x) -> Maybe .y
// mapMaybe f (Just x) = Just (f x)
// mapMaybe f Nothing  = Nothing

instance ==       (Maybe x) | == x
//	Nothing==Nothing
//	Just a ==Just b <= a==b