%
-% (c) The GRASP/AQUA Project, Glasgow University, 1992-1996
+% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[Maybes]{The `Maybe' types and associated utility functions}
\begin{code}
-#include "HsVersions.h"
-
module Maybes (
--- Maybe(..), -- no, it's in 1.3
+ Maybe2(..), Maybe3(..),
MaybeErr(..),
+ orElse,
mapMaybe,
allMaybes,
firstJust,
expectJust,
maybeToBool,
- assocMaybe,
- mkLookupFun, mkLookupFunDef,
+ thenMaybe, seqMaybe, returnMaybe, failMaybe, catMaybes,
+
+ thenMaB, returnMaB, failMaB
- failMaB,
- failMaybe,
- seqMaybe,
- returnMaB,
- returnMaybe,
- thenMaB,
- catMaybes
) where
-CHK_Ubiq() -- debugging consistency check
-import Unique (Unique) -- only for specialising
+#include "HsVersions.h"
-#if __GLASGOW_HASKELL__ >= 204
import Maybe( catMaybes, mapMaybe )
-#endif
+
+infixr 4 `orElse`
+\end{code}
+
+
+%************************************************************************
+%* *
+\subsection[Maybe2,3 types]{The @Maybe2@ and @Maybe3@ types}
+%* *
+%************************************************************************
+
+\begin{code}
+data Maybe2 a b = Just2 a b | Nothing2 deriving (Eq,Show)
+data Maybe3 a b c = Just3 a b c | Nothing3 deriving (Eq,Show)
\end{code}
Nothing -> Nothing
Just xs -> Just (x:xs)
-#if __GLASGOW_HASKELL__ < 204
- -- After 2.04 we get these from the library Maybe
-catMaybes :: [Maybe a] -> [a]
-catMaybes [] = []
-catMaybes (Nothing : xs) = catMaybes xs
-catMaybes (Just x : xs) = (x : catMaybes xs)
-
-mapMaybe :: (a -> Maybe b) -> [a] -> [b]
-mapMaybe f [] = []
-mapMaybe f (x:xs) = case f x of
- Just y -> y : mapMaybe f xs
- Nothing -> mapMaybe f xs
-#endif
\end{code}
@firstJust@ takes a list of @Maybes@ and returns the
seqMaybe (Just x) _ = Just x
seqMaybe Nothing my = my
+thenMaybe :: Maybe a -> (a -> Maybe b) -> Maybe b
+thenMaybe ma mb = case ma of
+ Just x -> mb x
+ Nothing -> Nothing
+
returnMaybe :: a -> Maybe a
returnMaybe = Just
failMaybe :: Maybe a
failMaybe = Nothing
-\end{code}
-
-Lookup functions
-~~~~~~~~~~~~~~~~
-
-@assocMaybe@ looks up in an assocation list, returning
-@Nothing@ if it fails.
-\begin{code}
-assocMaybe :: (Eq a) => [(a,b)] -> a -> Maybe b
-
-assocMaybe alist key
- = lookup alist
- where
- lookup [] = Nothing
- lookup ((tv,ty):rest) = if key == tv then Just ty else lookup rest
-
-{-# SPECIALIZE assocMaybe
- :: [(FAST_STRING, b)] -> FAST_STRING -> Maybe b
- , [(Int, b)] -> Int -> Maybe b
- , [(Unique, b)] -> Unique -> Maybe b
- #-}
+orElse :: Maybe a -> a -> a
+(Just x) `orElse` y = x
+Nothing `orElse` y = y
\end{code}
-@mkLookupFun eq alist@ is a function which looks up
-its argument in the association list @alist@, returning a Maybe type.
-@mkLookupFunDef@ is similar except that it is given a value to return
-on failure.
-
-\begin{code}
-mkLookupFun :: (key -> key -> Bool) -- Equality predicate
- -> [(key,val)] -- The assoc list
- -> key -- The key
- -> Maybe val -- The corresponding value
-
-mkLookupFun eq alist s
- = case [a | (s',a) <- alist, s' `eq` s] of
- [] -> Nothing
- (a:_) -> Just a
-
-mkLookupFunDef :: (key -> key -> Bool) -- Equality predicate
- -> [(key,val)] -- The assoc list
- -> val -- Value to return on failure
- -> key -- The key
- -> val -- The corresponding value
-
-mkLookupFunDef eq alist deflt s
- = case [a | (s',a) <- alist, s' `eq` s] of
- [] -> deflt
- (a:_) -> a
-\end{code}
%************************************************************************
%* *
failMaB :: err -> MaybeErr val err
failMaB e = Failed e
\end{code}
+