%
-% (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}
-#if defined(COMPILING_GHC)
-#include "HsVersions.h"
-#endif
-
module Maybes (
--- Maybe(..), -- no, it's in 1.3
- MaybeErr(..),
+ module Maybe, -- Re-export all of Maybe
+
+ MaybeErr(..), -- Instance of Monad
+ failME,
+ orElse,
+ mapCatMaybes,
allMaybes,
- catMaybes,
firstJust,
expectJust,
maybeToBool,
- assocMaybe,
- mkLookupFun, mkLookupFunDef,
-
- failMaB,
- failMaybe,
- seqMaybe,
- returnMaB,
- returnMaybe,
- thenMaB
-
-#if ! defined(COMPILING_GHC)
- , findJust
- , foldlMaybeErrs
- , listMaybeErrs
-#endif
+ thenMaybe, seqMaybe, returnMaybe, failMaybe
) where
-#if defined(COMPILING_GHC)
+#include "HsVersions.h"
-CHK_Ubiq() -- debugging consistency check
+import Maybe
-import Unique (Unique) -- only for specialising
-#endif
+infixr 4 `orElse`
\end{code}
-
%************************************************************************
%* *
\subsection[Maybe type]{The @Maybe@ type}
are any @Nothings@.
\begin{code}
-catMaybes :: [Maybe a] -> [a]
-catMaybes [] = []
-catMaybes (Nothing : xs) = catMaybes xs
-catMaybes (Just x : xs) = (x : catMaybes xs)
-
allMaybes :: [Maybe a] -> Maybe [a]
allMaybes [] = Just []
allMaybes (Nothing : ms) = Nothing
allMaybes (Just x : ms) = case (allMaybes ms) of
Nothing -> Nothing
Just xs -> Just (x:xs)
+
\end{code}
@firstJust@ takes a list of @Maybes@ and returns the
\end{code}
\begin{code}
-findJust :: (a -> Maybe b) -> [a] -> Maybe b
-findJust f [] = Nothing
-findJust f (a:as) = case f a of
- Nothing -> findJust f as
- b -> b
-\end{code}
-
-\begin{code}
expectJust :: String -> Maybe a -> a
{-# INLINE expectJust #-}
expectJust err (Just x) = x
expectJust err Nothing = error ("expectJust " ++ err)
\end{code}
+\begin{code}
+mapCatMaybes :: (a -> Maybe b) -> [a] -> [b]
+mapCatMaybes f [] = []
+mapCatMaybes f (x:xs) = case f x of
+ Just y -> y : mapCatMaybes f xs
+ Nothing -> mapCatMaybes f xs
+\end{code}
+
The Maybe monad
~~~~~~~~~~~~~~~
\begin{code}
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
-
-#if defined(COMPILING_GHC)
-{-# SPECIALIZE assocMaybe
- :: [(FAST_STRING, b)] -> FAST_STRING -> Maybe b
- , [(Int, b)] -> Int -> Maybe b
- , [(Unique, b)] -> Unique -> Maybe b
- , [(RdrName, b)] -> RdrName -> Maybe b
- #-}
-#endif
+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}
%************************************************************************
%* *
%************************************************************************
\begin{code}
-data MaybeErr val err = Succeeded val | Failed err
-\end{code}
-
-\begin{code}
-thenMaB :: MaybeErr val1 err -> (val1 -> MaybeErr val2 err) -> MaybeErr val2 err
-thenMaB m k
- = case m of
- Succeeded v -> k v
- Failed e -> Failed e
-
-returnMaB :: val -> MaybeErr val err
-returnMaB v = Succeeded v
-
-failMaB :: err -> MaybeErr val err
-failMaB e = Failed e
-\end{code}
-
-
-@listMaybeErrs@ takes a list of @MaybeErrs@ and, if they all succeed, returns
-a @Succeeded@ of a list of their values. If any fail, it returns a
-@Failed@ of the list of all the errors in the list.
-
-\begin{code}
-listMaybeErrs :: [MaybeErr val err] -> MaybeErr [val] [err]
-listMaybeErrs
- = foldr combine (Succeeded [])
- where
- combine (Succeeded v) (Succeeded vs) = Succeeded (v:vs)
- combine (Failed err) (Succeeded _) = Failed [err]
- combine (Succeeded v) (Failed errs) = Failed errs
- combine (Failed err) (Failed errs) = Failed (err:errs)
-\end{code}
+data MaybeErr err val = Succeeded val | Failed err
-@foldlMaybeErrs@ works along a list, carrying an accumulator; it
-applies the given function to the accumulator and the next list item,
-accumulating any errors that occur.
+instance Monad (MaybeErr err) where
+ return v = Succeeded v
+ Succeeded v >>= k = k v
+ Failed e >>= k = Failed e
-\begin{code}
-foldlMaybeErrs :: (acc -> input -> MaybeErr acc err)
- -> acc
- -> [input]
- -> MaybeErr acc [err]
-
-foldlMaybeErrs k accum ins = do_it [] accum ins
- where
- do_it [] acc [] = Succeeded acc
- do_it errs acc [] = Failed errs
- do_it errs acc (v:vs) = case (k acc v) of
- Succeeded acc' -> do_it errs acc' vs
- Failed err -> do_it (err:errs) acc vs
+failME :: err -> MaybeErr err val
+failME e = Failed e
\end{code}
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