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%
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1996
%
\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(..),
allMaybes,
firstJust,
expectJust,
maybeToBool,
assocMaybe,
mkLookupFun, mkLookupFunDef,
failMaB,
failMaybe,
seqMaybe,
returnMaB,
returnMaybe,
thenMaB
#if defined(COMPILING_GHC)
, catMaybes
#else
, findJust
, foldlMaybeErrs
, listMaybeErrs
#endif
) where
#if defined(COMPILING_GHC)
CHK_Ubiq() -- debugging consistency check
import Unique (Unique) -- only for specialising
#else
import Maybe -- renamer will tell us if there are any conflicts
#endif
\end{code}
%************************************************************************
%* *
\subsection[Maybe type]{The @Maybe@ type}
%* *
%************************************************************************
\begin{code}
maybeToBool :: Maybe a -> Bool
maybeToBool Nothing = False
maybeToBool (Just x) = True
\end{code}
@catMaybes@ takes a list of @Maybe@s and returns a list of
the contents of all the @Just@s in it. @allMaybes@ collects
a list of @Justs@ into a single @Just@, returning @Nothing@ if there
are any @Nothings@.
\begin{code}
#ifdef COMPILING_GHC
catMaybes :: [Maybe a] -> [a]
catMaybes [] = []
catMaybes (Nothing : xs) = catMaybes xs
catMaybes (Just x : xs) = (x : catMaybes xs)
#endif
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
first @Just@ if there is one, or @Nothing@ otherwise.
\begin{code}
firstJust :: [Maybe a] -> Maybe a
firstJust [] = Nothing
firstJust (Just x : ms) = Just x
firstJust (Nothing : ms) = firstJust ms
\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}
The Maybe monad
~~~~~~~~~~~~~~~
\begin{code}
seqMaybe :: Maybe a -> Maybe a -> Maybe a
seqMaybe (Just x) _ = Just x
seqMaybe Nothing my = my
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
\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}
%************************************************************************
%* *
\subsection[MaybeErr type]{The @MaybeErr@ type}
%* *
%************************************************************************
\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}
@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.
\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
\end{code}
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