path: root/compiler/utils/Util.hs

-- (c) The University of Glasgow 2006

{-# LANGUAGE CPP #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE BangPatterns #-}
#if __GLASGOW_HASKELL__ < 800
-- For CallStack business
{-# LANGUAGE ImplicitParams #-}
{-# LANGUAGE FlexibleContexts #-}
#endif

-- | Highly random utility functions
--
module Util (
        -- * Flags dependent on the compiler build
        ghciSupported, debugIsOn, ncgDebugIsOn,
        ghciTablesNextToCode,
        isWindowsHost, isDarwinHost,

        -- * General list processing
        zipEqual, zipWithEqual, zipWith3Equal, zipWith4Equal,
        zipLazy, stretchZipWith, zipWithAndUnzip,

        zipWithLazy, zipWith3Lazy,

        filterByList, filterByLists, partitionByList,

        unzipWith,

        mapFst, mapSnd, chkAppend,
        mapAndUnzip, mapAndUnzip3, mapAccumL2,
        nOfThem, filterOut, partitionWith, splitEithers,

        dropWhileEndLE, spanEnd,

        foldl1', foldl2, count, all2,

        lengthExceeds, lengthIs, lengthAtLeast,
        listLengthCmp, atLength,
        equalLength, compareLength, leLength,

        isSingleton, only, singleton,
        notNull, snocView,

        isIn, isn'tIn,

        chunkList,

        -- * Tuples
        fstOf3, sndOf3, thdOf3,
        firstM, first3M,
        fst3, snd3, third3,
        uncurry3,
        liftFst, liftSnd,

        -- * List operations controlled by another list
        takeList, dropList, splitAtList, split,
        dropTail,

        -- * For loop
        nTimes,

        -- * Sorting
        sortWith, minWith, nubSort,

        -- * Comparisons
        isEqual, eqListBy, eqMaybeBy,
        thenCmp, cmpList,
        removeSpaces,
        (<&&>), (<||>),

        -- * Edit distance
        fuzzyMatch, fuzzyLookup,

        -- * Transitive closures
        transitiveClosure,

        -- * Strictness
        seqList,

        -- * Module names
        looksLikeModuleName,
        looksLikePackageName,

        -- * Argument processing
        getCmd, toCmdArgs, toArgs,

        -- * Integers
        exactLog2,

        -- * Floating point
        readRational,

        -- * read helpers
        maybeRead, maybeReadFuzzy,

        -- * IO-ish utilities
        doesDirNameExist,
        getModificationUTCTime,
        modificationTimeIfExists,
        hSetTranslit,

        global, consIORef, globalM,

        -- * Filenames and paths
        Suffix,
        splitLongestPrefix,
        escapeSpaces,
        Direction(..), reslash,
        makeRelativeTo,

        -- * Utils for defining Data instances
        abstractConstr, abstractDataType, mkNoRepType,

        -- * Utils for printing C code
        charToC,

        -- * Hashing
        hashString,

        -- * Call stacks
        GHC.Stack.CallStack,
        HasCallStack,
        HasDebugCallStack,
        prettyCurrentCallStack,
    ) where

#include "HsVersions.h"

import Exception
import Panic

import Data.Data
import Data.IORef       ( IORef, newIORef, atomicModifyIORef' )
import System.IO.Unsafe ( unsafePerformIO )
import Data.List        hiding (group)

import GHC.Exts
import qualified GHC.Stack

import Control.Applicative ( liftA2 )
import Control.Monad    ( liftM )
import GHC.IO.Encoding (mkTextEncoding, textEncodingName)
import System.IO (Handle, hGetEncoding, hSetEncoding)
import System.IO.Error as IO ( isDoesNotExistError )
import System.Directory ( doesDirectoryExist, getModificationTime )
import System.FilePath

import Data.Char        ( isUpper, isAlphaNum, isSpace, chr, ord, isDigit )
import Data.Int
import Data.Ratio       ( (%) )
import Data.Ord         ( comparing )
import Data.Bits
import Data.Word
import qualified Data.IntMap as IM
import qualified Data.Set as Set

import Data.Time

infixr 9 `thenCmp`

{-
************************************************************************
*                                                                      *
\subsection{Is DEBUG on, are we on Windows, etc?}
*                                                                      *
************************************************************************

These booleans are global constants, set by CPP flags.  They allow us to
recompile a single module (this one) to change whether or not debug output
appears. They sometimes let us avoid even running CPP elsewhere.

It's important that the flags are literal constants (True/False). Then,
with -0, tests of the flags in other modules will simplify to the correct
branch of the conditional, thereby dropping debug code altogether when
the flags are off.
-}

ghciSupported :: Bool
#ifdef GHCI
ghciSupported = True
#else
ghciSupported = False
#endif

debugIsOn :: Bool
#ifdef DEBUG
debugIsOn = True
#else
debugIsOn = False
#endif

ncgDebugIsOn :: Bool
#ifdef NCG_DEBUG
ncgDebugIsOn = True
#else
ncgDebugIsOn = False
#endif

ghciTablesNextToCode :: Bool
#ifdef GHCI_TABLES_NEXT_TO_CODE
ghciTablesNextToCode = True
#else
ghciTablesNextToCode = False
#endif

isWindowsHost :: Bool
#ifdef mingw32_HOST_OS
isWindowsHost = True
#else
isWindowsHost = False
#endif

isDarwinHost :: Bool
#ifdef darwin_HOST_OS
isDarwinHost = True
#else
isDarwinHost = False
#endif

{-
************************************************************************
*                                                                      *
\subsection{A for loop}
*                                                                      *
************************************************************************
-}

-- | Compose a function with itself n times.  (nth rather than twice)
nTimes :: Int -> (a -> a) -> (a -> a)
nTimes 0 _ = id
nTimes 1 f = f
nTimes n f = f . nTimes (n-1) f

fstOf3   :: (a,b,c) -> a
sndOf3   :: (a,b,c) -> b
thdOf3   :: (a,b,c) -> c
fstOf3      (a,_,_) =  a
sndOf3      (_,b,_) =  b
thdOf3      (_,_,c) =  c

fst3 :: (a -> d) -> (a, b, c) -> (d, b, c)
fst3 f (a, b, c) = (f a, b, c)

snd3 :: (b -> d) -> (a, b, c) -> (a, d, c)
snd3 f (a, b, c) = (a, f b, c)

third3 :: (c -> d) -> (a, b, c) -> (a, b, d)
third3 f (a, b, c) = (a, b, f c)

uncurry3 :: (a -> b -> c -> d) -> (a, b, c) -> d
uncurry3 f (a, b, c) = f a b c

liftFst :: (a -> b) -> (a, c) -> (b, c)
liftFst f (a,c) = (f a, c)

liftSnd :: (a -> b) -> (c, a) -> (c, b)
liftSnd f (c,a) = (c, f a)

firstM :: Monad m => (a -> m c) -> (a, b) -> m (c, b)
firstM f (x, y) = liftM (\x' -> (x', y)) (f x)

first3M :: Monad m => (a -> m d) -> (a, b, c) -> m (d, b, c)
first3M f (x, y, z) = liftM (\x' -> (x', y, z)) (f x)

{-
************************************************************************
*                                                                      *
\subsection[Utils-lists]{General list processing}
*                                                                      *
************************************************************************
-}

filterOut :: (a->Bool) -> [a] -> [a]
-- ^ Like filter, only it reverses the sense of the test
filterOut _ [] = []
filterOut p (x:xs) | p x       = filterOut p xs
                   | otherwise = x : filterOut p xs

partitionWith :: (a -> Either b c) -> [a] -> ([b], [c])
-- ^ Uses a function to determine which of two output lists an input element should join
partitionWith _ [] = ([],[])
partitionWith f (x:xs) = case f x of
                         Left  b -> (b:bs, cs)
                         Right c -> (bs, c:cs)
    where (bs,cs) = partitionWith f xs

splitEithers :: [Either a b] -> ([a], [b])
-- ^ Teases a list of 'Either's apart into two lists
splitEithers [] = ([],[])
splitEithers (e : es) = case e of
                        Left x -> (x:xs, ys)
                        Right y -> (xs, y:ys)
    where (xs,ys) = splitEithers es

chkAppend :: [a] -> [a] -> [a]
-- Checks for the second arguemnt being empty
-- Used in situations where that situation is common
chkAppend xs ys
  | null ys   = xs
  | otherwise = xs ++ ys

{-
A paranoid @zip@ (and some @zipWith@ friends) that checks the lists
are of equal length.  Alastair Reid thinks this should only happen if
DEBUGging on; hey, why not?
-}

zipEqual        :: String -> [a] -> [b] -> [(a,b)]
zipWithEqual    :: String -> (a->b->c) -> [a]->[b]->[c]
zipWith3Equal   :: String -> (a->b->c->d) -> [a]->[b]->[c]->[d]
zipWith4Equal   :: String -> (a->b->c->d->e) -> [a]->[b]->[c]->[d]->[e]

#ifndef DEBUG
zipEqual      _ = zip
zipWithEqual  _ = zipWith
zipWith3Equal _ = zipWith3
zipWith4Equal _ = zipWith4
#else
zipEqual _   []     []     = []
zipEqual msg (a:as) (b:bs) = (a,b) : zipEqual msg as bs
zipEqual msg _      _      = panic ("zipEqual: unequal lists:"++msg)

zipWithEqual msg z (a:as) (b:bs)=  z a b : zipWithEqual msg z as bs
zipWithEqual _   _ [] []        =  []
zipWithEqual msg _ _ _          =  panic ("zipWithEqual: unequal lists:"++msg)

zipWith3Equal msg z (a:as) (b:bs) (c:cs)
                                =  z a b c : zipWith3Equal msg z as bs cs
zipWith3Equal _   _ [] []  []   =  []
zipWith3Equal msg _ _  _   _    =  panic ("zipWith3Equal: unequal lists:"++msg)

zipWith4Equal msg z (a:as) (b:bs) (c:cs) (d:ds)
                                =  z a b c d : zipWith4Equal msg z as bs cs ds
zipWith4Equal _   _ [] [] [] [] =  []
zipWith4Equal msg _ _  _  _  _  =  panic ("zipWith4Equal: unequal lists:"++msg)
#endif

-- | 'zipLazy' is a kind of 'zip' that is lazy in the second list (observe the ~)
zipLazy :: [a] -> [b] -> [(a,b)]
zipLazy []     _       = []
zipLazy (x:xs) ~(y:ys) = (x,y) : zipLazy xs ys

-- | 'zipWithLazy' is like 'zipWith' but is lazy in the second list.
-- The length of the output is always the same as the length of the first
-- list.
zipWithLazy :: (a -> b -> c) -> [a] -> [b] -> [c]
zipWithLazy _ []     _       = []
zipWithLazy f (a:as) ~(b:bs) = f a b : zipWithLazy f as bs

-- | 'zipWith3Lazy' is like 'zipWith3' but is lazy in the second and third lists.
-- The length of the output is always the same as the length of the first
-- list.
zipWith3Lazy :: (a -> b -> c -> d) -> [a] -> [b] -> [c] -> [d]
zipWith3Lazy _ []     _       _       = []
zipWith3Lazy f (a:as) ~(b:bs) ~(c:cs) = f a b c : zipWith3Lazy f as bs cs

-- | 'filterByList' takes a list of Bools and a list of some elements and
-- filters out these elements for which the corresponding value in the list of
-- Bools is False. This function does not check whether the lists have equal
-- length.
filterByList :: [Bool] -> [a] -> [a]
filterByList (True:bs)  (x:xs) = x : filterByList bs xs
filterByList (False:bs) (_:xs) =     filterByList bs xs
filterByList _          _      = []

-- | 'filterByLists' takes a list of Bools and two lists as input, and
-- outputs a new list consisting of elements from the last two input lists. For
-- each Bool in the list, if it is 'True', then it takes an element from the
-- former list. If it is 'False', it takes an element from the latter list.
-- The elements taken correspond to the index of the Bool in its list.
-- For example:
--
-- @
-- filterByLists [True, False, True, False] \"abcd\" \"wxyz\" = \"axcz\"
-- @
--
-- This function does not check whether the lists have equal length.
filterByLists :: [Bool] -> [a] -> [a] -> [a]
filterByLists (True:bs)  (x:xs) (_:ys) = x : filterByLists bs xs ys
filterByLists (False:bs) (_:xs) (y:ys) = y : filterByLists bs xs ys
filterByLists _          _      _      = []

-- | 'partitionByList' takes a list of Bools and a list of some elements and
-- partitions the list according to the list of Bools. Elements corresponding
-- to 'True' go to the left; elements corresponding to 'False' go to the right.
-- For example, @partitionByList [True, False, True] [1,2,3] == ([1,3], [2])@
-- This function does not check whether the lists have equal
-- length.
partitionByList :: [Bool] -> [a] -> ([a], [a])
partitionByList = go [] []
  where
    go trues falses (True  : bs) (x : xs) = go (x:trues) falses bs xs
    go trues falses (False : bs) (x : xs) = go trues (x:falses) bs xs
    go trues falses _ _ = (reverse trues, reverse falses)

stretchZipWith :: (a -> Bool) -> b -> (a->b->c) -> [a] -> [b] -> [c]
-- ^ @stretchZipWith p z f xs ys@ stretches @ys@ by inserting @z@ in
-- the places where @p@ returns @True@

stretchZipWith _ _ _ []     _ = []
stretchZipWith p z f (x:xs) ys
  | p x       = f x z : stretchZipWith p z f xs ys
  | otherwise = case ys of
                []     -> []
                (y:ys) -> f x y : stretchZipWith p z f xs ys

mapFst :: (a->c) -> [(a,b)] -> [(c,b)]
mapSnd :: (b->c) -> [(a,b)] -> [(a,c)]

mapFst f xys = [(f x, y) | (x,y) <- xys]
mapSnd f xys = [(x, f y) | (x,y) <- xys]

mapAndUnzip :: (a -> (b, c)) -> [a] -> ([b], [c])

mapAndUnzip _ [] = ([], [])
mapAndUnzip f (x:xs)
  = let (r1,  r2)  = f x
        (rs1, rs2) = mapAndUnzip f xs
    in
    (r1:rs1, r2:rs2)

mapAndUnzip3 :: (a -> (b, c, d)) -> [a] -> ([b], [c], [d])

mapAndUnzip3 _ [] = ([], [], [])
mapAndUnzip3 f (x:xs)
  = let (r1,  r2,  r3)  = f x
        (rs1, rs2, rs3) = mapAndUnzip3 f xs
    in
    (r1:rs1, r2:rs2, r3:rs3)

zipWithAndUnzip :: (a -> b -> (c,d)) -> [a] -> [b] -> ([c],[d])
zipWithAndUnzip f (a:as) (b:bs)
  = let (r1,  r2)  = f a b
        (rs1, rs2) = zipWithAndUnzip f as bs
    in
    (r1:rs1, r2:rs2)
zipWithAndUnzip _ _ _ = ([],[])

mapAccumL2 :: (s1 -> s2 -> a -> (s1, s2, b)) -> s1 -> s2 -> [a] -> (s1, s2, [b])
mapAccumL2 f s1 s2 xs = (s1', s2', ys)
  where ((s1', s2'), ys) = mapAccumL (\(s1, s2) x -> case f s1 s2 x of
                                                       (s1', s2', y) -> ((s1', s2'), y))
                                     (s1, s2) xs

nOfThem :: Int -> a -> [a]
nOfThem n thing = replicate n thing

-- | @atLength atLen atEnd ls n@ unravels list @ls@ to position @n@. Precisely:
--
-- @
--  atLength atLenPred atEndPred ls n
--   | n < 0         = atLenPred ls
--   | length ls < n = atEndPred (n - length ls)
--   | otherwise     = atLenPred (drop n ls)
-- @
atLength :: ([a] -> b)   -- Called when length ls >= n, passed (drop n ls)
                         --    NB: arg passed to this function may be []
         -> b            -- Called when length ls <  n
         -> [a]
         -> Int
         -> b
atLength atLenPred atEnd ls0 n0
  | n0 < 0    = atLenPred ls0
  | otherwise = go n0 ls0
  where
    -- go's first arg n >= 0
    go 0 ls     = atLenPred ls
    go _ []     = atEnd           -- n > 0 here
    go n (_:xs) = go (n-1) xs

-- Some special cases of atLength:

-- | @(lengthExceeds xs n) = (length xs > n)@
lengthExceeds :: [a] -> Int -> Bool
lengthExceeds lst n
  | n < 0
  = True
  | otherwise
  = atLength notNull False lst n

lengthAtLeast :: [a] -> Int -> Bool
lengthAtLeast = atLength (const True) False

-- | @(lengthIs xs n) = (length xs == n)@
lengthIs :: [a] -> Int -> Bool
lengthIs lst n
  | n < 0
  = False
  | otherwise
  = atLength null False lst n

listLengthCmp :: [a] -> Int -> Ordering
listLengthCmp = atLength atLen atEnd
 where
  atEnd = LT    -- Not yet seen 'n' elts, so list length is < n.

  atLen []     = EQ
  atLen _      = GT

equalLength :: [a] -> [b] -> Bool
equalLength []     []     = True
equalLength (_:xs) (_:ys) = equalLength xs ys
equalLength _      _      = False

compareLength :: [a] -> [b] -> Ordering
compareLength []     []     = EQ
compareLength (_:xs) (_:ys) = compareLength xs ys
compareLength []     _      = LT
compareLength _      []     = GT

leLength :: [a] -> [b] -> Bool
-- ^ True if length xs <= length ys
leLength xs ys = case compareLength xs ys of
                   LT -> True
                   EQ -> True
                   GT -> False

----------------------------
singleton :: a -> [a]
singleton x = [x]

isSingleton :: [a] -> Bool
isSingleton [_] = True
isSingleton _   = False

notNull :: [a] -> Bool
notNull [] = False
notNull _  = True

only :: [a] -> a
#ifdef DEBUG
only [a] = a
#else
only (a:_) = a
#endif
only _ = panic "Util: only"

-- Debugging/specialising versions of \tr{elem} and \tr{notElem}

isIn, isn'tIn :: Eq a => String -> a -> [a] -> Bool

# ifndef DEBUG
isIn    _msg x ys = x `elem` ys
isn'tIn _msg x ys = x `notElem` ys

# else /* DEBUG */
isIn msg x ys
  = elem100 0 x ys
  where
    elem100 :: Eq a => Int -> a -> [a] -> Bool
    elem100 _ _ [] = False
    elem100 i x (y:ys)
      | i > 100 = trace ("Over-long elem in " ++ msg) (x `elem` (y:ys))
      | otherwise = x == y || elem100 (i + 1) x ys

isn'tIn msg x ys
  = notElem100 0 x ys
  where
    notElem100 :: Eq a => Int -> a -> [a] -> Bool
    notElem100 _ _ [] =  True
    notElem100 i x (y:ys)
      | i > 100 = trace ("Over-long notElem in " ++ msg) (x `notElem` (y:ys))
      | otherwise = x /= y && notElem100 (i + 1) x ys
# endif /* DEBUG */


-- | Split a list into chunks of /n/ elements
chunkList :: Int -> [a] -> [[a]]
chunkList _ [] = []
chunkList n xs = as : chunkList n bs where (as,bs) = splitAt n xs

{-
************************************************************************
*                                                                      *
\subsubsection{Sort utils}
*                                                                      *
************************************************************************
-}

minWith :: Ord b => (a -> b) -> [a] -> a
minWith get_key xs = ASSERT( not (null xs) )
                     head (sortWith get_key xs)

nubSort :: Ord a => [a] -> [a]
nubSort = Set.toAscList . Set.fromList

{-
************************************************************************
*                                                                      *
\subsection[Utils-transitive-closure]{Transitive closure}
*                                                                      *
************************************************************************

This algorithm for transitive closure is straightforward, albeit quadratic.
-}

transitiveClosure :: (a -> [a])         -- Successor function
                  -> (a -> a -> Bool)   -- Equality predicate
                  -> [a]
                  -> [a]                -- The transitive closure

transitiveClosure succ eq xs
 = go [] xs
 where
   go done []                      = done
   go done (x:xs) | x `is_in` done = go done xs
                  | otherwise      = go (x:done) (succ x ++ xs)

   _ `is_in` []                 = False
   x `is_in` (y:ys) | eq x y    = True
                    | otherwise = x `is_in` ys

{-
************************************************************************
*                                                                      *
\subsection[Utils-accum]{Accumulating}
*                                                                      *
************************************************************************

A combination of foldl with zip.  It works with equal length lists.
-}

foldl2 :: (acc -> a -> b -> acc) -> acc -> [a] -> [b] -> acc
foldl2 _ z [] [] = z
foldl2 k z (a:as) (b:bs) = foldl2 k (k z a b) as bs
foldl2 _ _ _      _      = panic "Util: foldl2"

all2 :: (a -> b -> Bool) -> [a] -> [b] -> Bool
-- True if the lists are the same length, and
-- all corresponding elements satisfy the predicate
all2 _ []     []     = True
all2 p (x:xs) (y:ys) = p x y && all2 p xs ys
all2 _ _      _      = False

-- Count the number of times a predicate is true

count :: (a -> Bool) -> [a] -> Int
count p = go 0
  where go !n [] = n
        go !n (x:xs) | p x       = go (n+1) xs
                     | otherwise = go n xs

{-
@splitAt@, @take@, and @drop@ but with length of another
list giving the break-off point:
-}

takeList :: [b] -> [a] -> [a]
-- (takeList as bs) trims bs to the be same length
-- as as, unless as is longer in which case it's a no-op
takeList [] _ = []
takeList (_:xs) ls =
   case ls of
     [] -> []
     (y:ys) -> y : takeList xs ys

dropList :: [b] -> [a] -> [a]
dropList [] xs    = xs
dropList _  xs@[] = xs
dropList (_:xs) (_:ys) = dropList xs ys


splitAtList :: [b] -> [a] -> ([a], [a])
splitAtList [] xs     = ([], xs)
splitAtList _ xs@[]   = (xs, xs)
splitAtList (_:xs) (y:ys) = (y:ys', ys'')
    where
      (ys', ys'') = splitAtList xs ys

-- drop from the end of a list
dropTail :: Int -> [a] -> [a]
-- Specification: dropTail n = reverse . drop n . reverse
-- Better implemention due to Joachim Breitner
-- http://www.joachim-breitner.de/blog/archives/600-On-taking-the-last-n-elements-of-a-list.html
dropTail n xs
  = go (drop n xs) xs
  where
    go (_:ys) (x:xs) = x : go ys xs
    go _      _      = []  -- Stop when ys runs out
                           -- It'll always run out before xs does

-- dropWhile from the end of a list. This is similar to Data.List.dropWhileEnd,
-- but is lazy in the elements and strict in the spine. For reasonably short lists,
-- such as path names and typical lines of text, dropWhileEndLE is generally
-- faster than dropWhileEnd. Its advantage is magnified when the predicate is
-- expensive--using dropWhileEndLE isSpace to strip the space off a line of text
-- is generally much faster than using dropWhileEnd isSpace for that purpose.
-- Specification: dropWhileEndLE p = reverse . dropWhile p . reverse
-- Pay attention to the short-circuit (&&)! The order of its arguments is the only
-- difference between dropWhileEnd and dropWhileEndLE.
dropWhileEndLE :: (a -> Bool) -> [a] -> [a]
dropWhileEndLE p = foldr (\x r -> if null r && p x then [] else x:r) []

-- | @spanEnd p l == reverse (span p (reverse l))@. The first list
-- returns actually comes after the second list (when you look at the
-- input list).
spanEnd :: (a -> Bool) -> [a] -> ([a], [a])
spanEnd p l = go l [] [] l
  where go yes _rev_yes rev_no [] = (yes, reverse rev_no)
        go yes rev_yes  rev_no (x:xs)
          | p x       = go yes (x : rev_yes) rev_no                  xs
          | otherwise = go xs  []            (x : rev_yes ++ rev_no) xs


snocView :: [a] -> Maybe ([a],a)
        -- Split off the last element
snocView [] = Nothing
snocView xs = go [] xs
            where
                -- Invariant: second arg is non-empty
              go acc [x]    = Just (reverse acc, x)
              go acc (x:xs) = go (x:acc) xs
              go _   []     = panic "Util: snocView"

split :: Char -> String -> [String]
split c s = case rest of
                []     -> [chunk]
                _:rest -> chunk : split c rest
  where (chunk, rest) = break (==c) s

{-
************************************************************************
*                                                                      *
\subsection[Utils-comparison]{Comparisons}
*                                                                      *
************************************************************************
-}

isEqual :: Ordering -> Bool
-- Often used in (isEqual (a `compare` b))
isEqual GT = False
isEqual EQ = True
isEqual LT = False

thenCmp :: Ordering -> Ordering -> Ordering
{-# INLINE thenCmp #-}
thenCmp EQ       ordering = ordering
thenCmp ordering _        = ordering

eqListBy :: (a->a->Bool) -> [a] -> [a] -> Bool
eqListBy _  []     []     = True
eqListBy eq (x:xs) (y:ys) = eq x y && eqListBy eq xs ys
eqListBy _  _      _      = False

eqMaybeBy :: (a ->a->Bool) -> Maybe a -> Maybe a -> Bool
eqMaybeBy _  Nothing  Nothing  = True
eqMaybeBy eq (Just x) (Just y) = eq x y
eqMaybeBy _  _        _        = False

cmpList :: (a -> a -> Ordering) -> [a] -> [a] -> Ordering
    -- `cmpList' uses a user-specified comparer

cmpList _   []     [] = EQ
cmpList _   []     _  = LT
cmpList _   _      [] = GT
cmpList cmp (a:as) (b:bs)
  = case cmp a b of { EQ -> cmpList cmp as bs; xxx -> xxx }

removeSpaces :: String -> String
removeSpaces = dropWhileEndLE isSpace . dropWhile isSpace

-- Boolean operators lifted to Applicative
(<&&>) :: Applicative f => f Bool -> f Bool -> f Bool
(<&&>) = liftA2 (&&)
infixr 3 <&&> -- same as (&&)

(<||>) :: Applicative f => f Bool -> f Bool -> f Bool
(<||>) = liftA2 (||)
infixr 2 <||> -- same as (||)

{-
************************************************************************
*                                                                      *
\subsection{Edit distance}
*                                                                      *
************************************************************************
-}

-- | Find the "restricted" Damerau-Levenshtein edit distance between two strings.
-- See: <http://en.wikipedia.org/wiki/Damerau-Levenshtein_distance>.
-- Based on the algorithm presented in "A Bit-Vector Algorithm for Computing
-- Levenshtein and Damerau Edit Distances" in PSC'02 (Heikki Hyyro).
-- See http://www.cs.uta.fi/~helmu/pubs/psc02.pdf and
--     http://www.cs.uta.fi/~helmu/pubs/PSCerr.html for an explanation
restrictedDamerauLevenshteinDistance :: String -> String -> Int
restrictedDamerauLevenshteinDistance str1 str2
  = restrictedDamerauLevenshteinDistanceWithLengths m n str1 str2
  where
    m = length str1
    n = length str2

restrictedDamerauLevenshteinDistanceWithLengths
  :: Int -> Int -> String -> String -> Int
restrictedDamerauLevenshteinDistanceWithLengths m n str1 str2
  | m <= n
  = if n <= 32 -- n must be larger so this check is sufficient
    then restrictedDamerauLevenshteinDistance' (undefined :: Word32) m n str1 str2
    else restrictedDamerauLevenshteinDistance' (undefined :: Integer) m n str1 str2

  | otherwise
  = if m <= 32 -- m must be larger so this check is sufficient
    then restrictedDamerauLevenshteinDistance' (undefined :: Word32) n m str2 str1
    else restrictedDamerauLevenshteinDistance' (undefined :: Integer) n m str2 str1

restrictedDamerauLevenshteinDistance'
  :: (Bits bv, Num bv) => bv -> Int -> Int -> String -> String -> Int
restrictedDamerauLevenshteinDistance' _bv_dummy m n str1 str2
  | [] <- str1 = n
  | otherwise  = extractAnswer $
                 foldl' (restrictedDamerauLevenshteinDistanceWorker
                             (matchVectors str1) top_bit_mask vector_mask)
                        (0, 0, m_ones, 0, m) str2
  where
    m_ones@vector_mask = (2 ^ m) - 1
    top_bit_mask = (1 `shiftL` (m - 1)) `asTypeOf` _bv_dummy
    extractAnswer (_, _, _, _, distance) = distance

restrictedDamerauLevenshteinDistanceWorker
      :: (Bits bv, Num bv) => IM.IntMap bv -> bv -> bv
      -> (bv, bv, bv, bv, Int) -> Char -> (bv, bv, bv, bv, Int)
restrictedDamerauLevenshteinDistanceWorker str1_mvs top_bit_mask vector_mask
                                           (pm, d0, vp, vn, distance) char2
  = seq str1_mvs $ seq top_bit_mask $ seq vector_mask $
    seq pm' $ seq d0' $ seq vp' $ seq vn' $
    seq distance'' $ seq char2 $
    (pm', d0', vp', vn', distance'')
  where
    pm' = IM.findWithDefault 0 (ord char2) str1_mvs

    d0' = ((((sizedComplement vector_mask d0) .&. pm') `shiftL` 1) .&. pm)
      .|. ((((pm' .&. vp) + vp) .&. vector_mask) `xor` vp) .|. pm' .|. vn
          -- No need to mask the shiftL because of the restricted range of pm

    hp' = vn .|. sizedComplement vector_mask (d0' .|. vp)
    hn' = d0' .&. vp

    hp'_shift = ((hp' `shiftL` 1) .|. 1) .&. vector_mask
    hn'_shift = (hn' `shiftL` 1) .&. vector_mask
    vp' = hn'_shift .|. sizedComplement vector_mask (d0' .|. hp'_shift)
    vn' = d0' .&. hp'_shift

    distance' = if hp' .&. top_bit_mask /= 0 then distance + 1 else distance
    distance'' = if hn' .&. top_bit_mask /= 0 then distance' - 1 else distance'

sizedComplement :: Bits bv => bv -> bv -> bv
sizedComplement vector_mask vect = vector_mask `xor` vect

matchVectors :: (Bits bv, Num bv) => String -> IM.IntMap bv
matchVectors = snd . foldl' go (0 :: Int, IM.empty)
  where
    go (ix, im) char = let ix' = ix + 1
                           im' = IM.insertWith (.|.) (ord char) (2 ^ ix) im
                       in seq ix' $ seq im' $ (ix', im')

{-# SPECIALIZE INLINE restrictedDamerauLevenshteinDistance'
                      :: Word32 -> Int -> Int -> String -> String -> Int #-}
{-# SPECIALIZE INLINE restrictedDamerauLevenshteinDistance'
                      :: Integer -> Int -> Int -> String -> String -> Int #-}

{-# SPECIALIZE restrictedDamerauLevenshteinDistanceWorker
               :: IM.IntMap Word32 -> Word32 -> Word32
               -> (Word32, Word32, Word32, Word32, Int)
               -> Char -> (Word32, Word32, Word32, Word32, Int) #-}
{-# SPECIALIZE restrictedDamerauLevenshteinDistanceWorker
               :: IM.IntMap Integer -> Integer -> Integer
               -> (Integer, Integer, Integer, Integer, Int)
               -> Char -> (Integer, Integer, Integer, Integer, Int) #-}

{-# SPECIALIZE INLINE sizedComplement :: Word32 -> Word32 -> Word32 #-}
{-# SPECIALIZE INLINE sizedComplement :: Integer -> Integer -> Integer #-}

{-# SPECIALIZE matchVectors :: String -> IM.IntMap Word32 #-}
{-# SPECIALIZE matchVectors :: String -> IM.IntMap Integer #-}

fuzzyMatch :: String -> [String] -> [String]
fuzzyMatch key vals = fuzzyLookup key [(v,v) | v <- vals]

-- | Search for possible matches to the users input in the given list,
-- returning a small number of ranked results
fuzzyLookup :: String -> [(String,a)] -> [a]
fuzzyLookup user_entered possibilites
  = map fst $ take mAX_RESULTS $ sortBy (comparing snd)
    [ (poss_val, distance) | (poss_str, poss_val) <- possibilites
                       , let distance = restrictedDamerauLevenshteinDistance
                                            poss_str user_entered
                       , distance <= fuzzy_threshold ]
  where
    -- Work out an approriate match threshold:
    -- We report a candidate if its edit distance is <= the threshold,
    -- The threshhold is set to about a quarter of the # of characters the user entered
    --   Length    Threshold
    --     1         0          -- Don't suggest *any* candidates
    --     2         1          -- for single-char identifiers
    --     3         1
    --     4         1
    --     5         1
    --     6         2
    --
    fuzzy_threshold = truncate $ fromIntegral (length user_entered + 2) / (4 :: Rational)
    mAX_RESULTS = 3

{-
************************************************************************
*                                                                      *
\subsection[Utils-pairs]{Pairs}
*                                                                      *
************************************************************************
-}

unzipWith :: (a -> b -> c) -> [(a, b)] -> [c]
unzipWith f pairs = map ( \ (a, b) -> f a b ) pairs

seqList :: [a] -> b -> b
seqList [] b = b
seqList (x:xs) b = x `seq` seqList xs b

-- Global variables:

global :: a -> IORef a
global a = unsafePerformIO (newIORef a)

consIORef :: IORef [a] -> a -> IO ()
consIORef var x = do
  atomicModifyIORef' var (\xs -> (x:xs,()))

globalM :: IO a -> IORef a
globalM ma = unsafePerformIO (ma >>= newIORef)

-- Module names:

looksLikeModuleName :: String -> Bool
looksLikeModuleName [] = False
looksLikeModuleName (c:cs) = isUpper c && go cs
  where go [] = True
        go ('.':cs) = looksLikeModuleName cs
        go (c:cs)   = (isAlphaNum c || c == '_' || c == '\'') && go cs

-- Similar to 'parse' for Distribution.Package.PackageName,
-- but we don't want to depend on Cabal.
looksLikePackageName :: String -> Bool
looksLikePackageName = all (all isAlphaNum <&&> not . (all isDigit)) . split '-'

{-
Akin to @Prelude.words@, but acts like the Bourne shell, treating
quoted strings as Haskell Strings, and also parses Haskell [String]
syntax.
-}

getCmd :: String -> Either String             -- Error
                           (String, String) -- (Cmd, Rest)
getCmd s = case break isSpace $ dropWhile isSpace s of
           ([], _) -> Left ("Couldn't find command in " ++ show s)
           res -> Right res

toCmdArgs :: String -> Either String             -- Error
                              (String, [String]) -- (Cmd, Args)
toCmdArgs s = case getCmd s of
              Left err -> Left err
              Right (cmd, s') -> case toArgs s' of
                                 Left err -> Left err
                                 Right args -> Right (cmd, args)

toArgs :: String -> Either String   -- Error
                           [String] -- Args
toArgs str
    = case dropWhile isSpace str of
      s@('[':_) -> case reads s of
                   [(args, spaces)]
                    | all isSpace spaces ->
                       Right args
                   _ ->
                       Left ("Couldn't read " ++ show str ++ " as [String]")
      s -> toArgs' s
 where
  toArgs' :: String -> Either String [String]
  -- Remove outer quotes:
  -- > toArgs' "\"foo\" \"bar baz\""
  -- Right ["foo", "bar baz"]
  --
  -- Keep inner quotes:
  -- > toArgs' "-DFOO=\"bar baz\""
  -- Right ["-DFOO=\"bar baz\""]
  toArgs' s = case dropWhile isSpace s of
              [] -> Right []
              ('"' : _) -> do
                    -- readAsString removes outer quotes
                    (arg, rest) <- readAsString s
                    (arg:) `fmap` toArgs' rest
              s' -> case break (isSpace <||> (== '"')) s' of
                    (argPart1, s''@('"':_)) -> do
                        (argPart2, rest) <- readAsString s''
                        -- show argPart2 to keep inner quotes
                        ((argPart1 ++ show argPart2):) `fmap` toArgs' rest
                    (arg, s'') -> (arg:) `fmap` toArgs' s''

  readAsString :: String -> Either String (String, String)
  readAsString s = case reads s of
                [(arg, rest)]
                    -- rest must either be [] or start with a space
                    | all isSpace (take 1 rest) ->
                    Right (arg, rest)
                _ ->
                    Left ("Couldn't read " ++ show s ++ " as String")
-----------------------------------------------------------------------------
-- Integers

-- This algorithm for determining the $\log_2$ of exact powers of 2 comes
-- from GCC.  It requires bit manipulation primitives, and we use GHC
-- extensions.  Tough.

exactLog2 :: Integer -> Maybe Integer
exactLog2 x
  = if (x <= 0 || x >= 2147483648) then
       Nothing
    else
       if (x .&. (-x)) /= x then
          Nothing
       else
          Just (pow2 x)
  where
    pow2 x | x == 1 = 0
           | otherwise = 1 + pow2 (x `shiftR` 1)


{-
-- -----------------------------------------------------------------------------
-- Floats
-}

readRational__ :: ReadS Rational -- NB: doesn't handle leading "-"
readRational__ r = do
     (n,d,s) <- readFix r
     (k,t)   <- readExp s
     return ((n%1)*10^^(k-d), t)
 where
     readFix r = do
        (ds,s)  <- lexDecDigits r
        (ds',t) <- lexDotDigits s
        return (read (ds++ds'), length ds', t)

     readExp (e:s) | e `elem` "eE" = readExp' s
     readExp s                     = return (0,s)

     readExp' ('+':s) = readDec s
     readExp' ('-':s) = do (k,t) <- readDec s
                           return (-k,t)
     readExp' s       = readDec s

     readDec s = do
        (ds,r) <- nonnull isDigit s
        return (foldl1 (\n d -> n * 10 + d) [ ord d - ord '0' | d <- ds ],
                r)

     lexDecDigits = nonnull isDigit

     lexDotDigits ('.':s) = return (span isDigit s)
     lexDotDigits s       = return ("",s)

     nonnull p s = do (cs@(_:_),t) <- return (span p s)
                      return (cs,t)

readRational :: String -> Rational -- NB: *does* handle a leading "-"
readRational top_s
  = case top_s of
      '-' : xs -> - (read_me xs)
      xs       -> read_me xs
  where
    read_me s
      = case (do { (x,"") <- readRational__ s ; return x }) of
          [x] -> x
          []  -> error ("readRational: no parse:"        ++ top_s)
          _   -> error ("readRational: ambiguous parse:" ++ top_s)


-----------------------------------------------------------------------------
-- read helpers

maybeRead :: Read a => String -> Maybe a
maybeRead str = case reads str of
                [(x, "")] -> Just x
                _         -> Nothing

maybeReadFuzzy :: Read a => String -> Maybe a
maybeReadFuzzy str = case reads str of
                     [(x, s)]
                      | all isSpace s ->
                         Just x
                     _ ->
                         Nothing

-----------------------------------------------------------------------------
-- Verify that the 'dirname' portion of a FilePath exists.
--
doesDirNameExist :: FilePath -> IO Bool
doesDirNameExist fpath = doesDirectoryExist (takeDirectory fpath)

-----------------------------------------------------------------------------
-- Backwards compatibility definition of getModificationTime

getModificationUTCTime :: FilePath -> IO UTCTime
getModificationUTCTime = getModificationTime

-- --------------------------------------------------------------
-- check existence & modification time at the same time

modificationTimeIfExists :: FilePath -> IO (Maybe UTCTime)
modificationTimeIfExists f = do
  (do t <- getModificationUTCTime f; return (Just t))
        `catchIO` \e -> if isDoesNotExistError e
                        then return Nothing
                        else ioError e

-- --------------------------------------------------------------
-- Change the character encoding of the given Handle to transliterate
-- on unsupported characters instead of throwing an exception

hSetTranslit :: Handle -> IO ()
hSetTranslit h = do
    menc <- hGetEncoding h
    case fmap textEncodingName menc of
        Just name | '/' `notElem` name -> do
            enc' <- mkTextEncoding $ name ++ "//TRANSLIT"
            hSetEncoding h enc'
        _ -> return ()

-- split a string at the last character where 'pred' is True,
-- returning a pair of strings. The first component holds the string
-- up (but not including) the last character for which 'pred' returned
-- True, the second whatever comes after (but also not including the
-- last character).
--
-- If 'pred' returns False for all characters in the string, the original
-- string is returned in the first component (and the second one is just
-- empty).
splitLongestPrefix :: String -> (Char -> Bool) -> (String,String)
splitLongestPrefix str pred
  | null r_pre = (str,           [])
  | otherwise  = (reverse (tail r_pre), reverse r_suf)
                           -- 'tail' drops the char satisfying 'pred'
  where (r_suf, r_pre) = break pred (reverse str)

escapeSpaces :: String -> String
escapeSpaces = foldr (\c s -> if isSpace c then '\\':c:s else c:s) ""

type Suffix = String

--------------------------------------------------------------
-- * Search path
--------------------------------------------------------------

data Direction = Forwards | Backwards

reslash :: Direction -> FilePath -> FilePath
reslash d = f
    where f ('/'  : xs) = slash : f xs
          f ('\\' : xs) = slash : f xs
          f (x    : xs) = x     : f xs
          f ""          = ""
          slash = case d of
                  Forwards -> '/'
                  Backwards -> '\\'

makeRelativeTo :: FilePath -> FilePath -> FilePath
this `makeRelativeTo` that = directory </> thisFilename
    where (thisDirectory, thisFilename) = splitFileName this
          thatDirectory = dropFileName that
          directory = joinPath $ f (splitPath thisDirectory)
                                   (splitPath thatDirectory)

          f (x : xs) (y : ys)
           | x == y = f xs ys
          f xs ys = replicate (length ys) ".." ++ xs

{-
************************************************************************
*                                                                      *
\subsection[Utils-Data]{Utils for defining Data instances}
*                                                                      *
************************************************************************

These functions helps us to define Data instances for abstract types.
-}

abstractConstr :: String -> Constr
abstractConstr n = mkConstr (abstractDataType n) ("{abstract:"++n++"}") [] Prefix

abstractDataType :: String -> DataType
abstractDataType n = mkDataType n [abstractConstr n]

{-
************************************************************************
*                                                                      *
\subsection[Utils-C]{Utils for printing C code}
*                                                                      *
************************************************************************
-}

charToC :: Word8 -> String
charToC w =
  case chr (fromIntegral w) of
        '\"' -> "\\\""
        '\'' -> "\\\'"
        '\\' -> "\\\\"
        c | c >= ' ' && c <= '~' -> [c]
          | otherwise -> ['\\',
                         chr (ord '0' + ord c `div` 64),
                         chr (ord '0' + ord c `div` 8 `mod` 8),
                         chr (ord '0' + ord c         `mod` 8)]

{-
************************************************************************
*                                                                      *
\subsection[Utils-Hashing]{Utils for hashing}
*                                                                      *
************************************************************************
-}

-- | A sample hash function for Strings.  We keep multiplying by the
-- golden ratio and adding.  The implementation is:
--
-- > hashString = foldl' f golden
-- >   where f m c = fromIntegral (ord c) * magic + hashInt32 m
-- >         magic = 0xdeadbeef
--
-- Where hashInt32 works just as hashInt shown above.
--
-- Knuth argues that repeated multiplication by the golden ratio
-- will minimize gaps in the hash space, and thus it's a good choice
-- for combining together multiple keys to form one.
--
-- Here we know that individual characters c are often small, and this
-- produces frequent collisions if we use ord c alone.  A
-- particular problem are the shorter low ASCII and ISO-8859-1
-- character strings.  We pre-multiply by a magic twiddle factor to
-- obtain a good distribution.  In fact, given the following test:
--
-- > testp :: Int32 -> Int
-- > testp k = (n - ) . length . group . sort . map hs . take n $ ls
-- >   where ls = [] : [c : l | l <- ls, c <- ['\0'..'\xff']]
-- >         hs = foldl' f golden
-- >         f m c = fromIntegral (ord c) * k + hashInt32 m
-- >         n = 100000
--
-- We discover that testp magic = 0.
hashString :: String -> Int32
hashString = foldl' f golden
   where f m c = fromIntegral (ord c) * magic + hashInt32 m
         magic = fromIntegral (0xdeadbeef :: Word32)

golden :: Int32
golden = 1013904242 -- = round ((sqrt 5 - 1) * 2^32) :: Int32
-- was -1640531527 = round ((sqrt 5 - 1) * 2^31) :: Int32
-- but that has bad mulHi properties (even adding 2^32 to get its inverse)
-- Whereas the above works well and contains no hash duplications for
-- [-32767..65536]

-- | A sample (and useful) hash function for Int32,
-- implemented by extracting the uppermost 32 bits of the 64-bit
-- result of multiplying by a 33-bit constant.  The constant is from
-- Knuth, derived from the golden ratio:
--
-- > golden = round ((sqrt 5 - 1) * 2^32)
--
-- We get good key uniqueness on small inputs
-- (a problem with previous versions):
--  (length $ group $ sort $ map hashInt32 [-32767..65536]) == 65536 + 32768
--
hashInt32 :: Int32 -> Int32
hashInt32 x = mulHi x golden + x

-- hi 32 bits of a x-bit * 32 bit -> 64-bit multiply
mulHi :: Int32 -> Int32 -> Int32
mulHi a b = fromIntegral (r `shiftR` 32)
   where r :: Int64
         r = fromIntegral a * fromIntegral b

-- | A compatibility wrapper for the @GHC.Stack.HasCallStack@ constraint.
#if __GLASGOW_HASKELL__ >= 800
type HasCallStack = GHC.Stack.HasCallStack
#elif MIN_VERSION_GLASGOW_HASKELL(7,10,2,0)
type HasCallStack = (?callStack :: GHC.Stack.CallStack)
-- CallStack wasn't present in GHC 7.10.1, disable callstacks in stage 1
#else
type HasCallStack = (() :: Constraint)
#endif

-- | A call stack constraint, but only when 'isDebugOn'.
#if DEBUG
type HasDebugCallStack = HasCallStack
#else
type HasDebugCallStack = (() :: Constraint)
#endif

-- | Pretty-print the current callstack
#if __GLASGOW_HASKELL__ >= 800
prettyCurrentCallStack :: HasCallStack => String
prettyCurrentCallStack = GHC.Stack.prettyCallStack GHC.Stack.callStack
#elif MIN_VERSION_GLASGOW_HASKELL(7,10,2,0)
prettyCurrentCallStack :: (?callStack :: GHC.Stack.CallStack) => String
prettyCurrentCallStack = GHC.Stack.showCallStack ?callStack
#else
prettyCurrentCallStack :: HasCallStack => String
prettyCurrentCallStack = "Call stack unavailable"
#endif