Util

Highly random utility functions

Flags dependent on the compiler build

ghciSupported :: Bool

debugIsOn :: Bool

ncgDebugIsOn :: Bool

ghciTablesNextToCode :: Bool

isWindowsHost :: Bool

isDarwinHost :: Bool

General list processing

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]

zipLazy :: [a] -> [b] -> [(a, b)]

zipLazy is a kind of zip that is lazy in the second list (observe the ~)

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

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

zipWithLazy :: (a -> b -> c) -> [a] -> [b] -> [c]

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.

zipWith3Lazy :: (a -> b -> c -> d) -> [a] -> [b] -> [c] -> [d]

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.

filterByList :: [Bool] -> [a] -> [a]

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.

filterByLists :: [Bool] -> [a] -> [a] -> [a]

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.

partitionByList :: [Bool] -> [a] -> ([a], [a])

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.

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

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

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

chkAppend :: [a] -> [a] -> [a]

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

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

mapAccumL2 :: (s1 -> s2 -> a -> (s1, s2, b)) -> s1 -> s2 -> [a] -> (s1, s2, [b])

nOfThem :: Int -> a -> [a]

filterOut :: (a -> Bool) -> [a] -> [a]

Like filter, only it reverses the sense of the test

partitionWith :: (a -> Either b c) -> [a] -> ([b], [c])

Uses a function to determine which of two output lists an input element should join

splitEithers :: [Either a b] -> ([a], [b])

Teases a list of Eithers apart into two lists

dropWhileEndLE :: (a -> Bool) -> [a] -> [a]

spanEnd :: (a -> Bool) -> [a] -> ([a], [a])

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).

foldl1' :: (a -> a -> a) -> [a] -> a

A strict version of foldl1

foldl2 :: (acc -> a -> b -> acc) -> acc -> [a] -> [b] -> acc

count :: (a -> Bool) -> [a] -> Int

all2 :: (a -> b -> Bool) -> [a] -> [b] -> Bool

lengthExceeds :: [a] -> Int -> Bool

(lengthExceeds xs n) = (length xs > n)

lengthIs :: [a] -> Int -> Bool

(lengthIs xs n) = (length xs == n)

lengthAtLeast :: [a] -> Int -> Bool

listLengthCmp :: [a] -> Int -> Ordering

atLength :: ([a] -> b) -> b -> [a] -> Int -> b

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)

equalLength :: [a] -> [b] -> Bool

compareLength :: [a] -> [b] -> Ordering

leLength :: [a] -> [b] -> Bool

True if length xs <= length ys

isSingleton :: [a] -> Bool

only :: [a] -> a

singleton :: a -> [a]

notNull :: [a] -> Bool

snocView :: [a] -> Maybe ([a], a)

isIn :: Eq a => String -> a -> [a] -> Bool

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

chunkList :: Int -> [a] -> [[a]]

Split a list into chunks of n elements

changeLast :: [a] -> a -> [a]

Replace the last element of a list with another element.

Tuples

fstOf3 :: (a, b, c) -> a

sndOf3 :: (a, b, c) -> b

thdOf3 :: (a, b, c) -> c

firstM :: Monad m => (a -> m c) -> (a, b) -> m (c, b)

first3M :: Monad m => (a -> m d) -> (a, b, c) -> m (d, b, c)

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

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

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

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

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

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

List operations controlled by another list

takeList :: [b] -> [a] -> [a]

dropList :: [b] -> [a] -> [a]

splitAtList :: [b] -> [a] -> ([a], [a])

split :: Char -> String -> [String]

dropTail :: Int -> [a] -> [a]

capitalise :: String -> String

Convert a word to title case by capitalising the first letter

For loop

nTimes :: Int -> (a -> a) -> a -> a

Compose a function with itself n times. (nth rather than twice)

Sorting

sortWith :: Ord b => (a -> b) -> [a] -> [a]

The sortWith function sorts a list of elements using the user supplied function to project something out of each element

minWith :: Ord b => (a -> b) -> [a] -> a

nubSort :: Ord a => [a] -> [a]

Comparisons

isEqual :: Ordering -> Bool

eqListBy :: (a -> a -> Bool) -> [a] -> [a] -> Bool

eqMaybeBy :: (a -> a -> Bool) -> Maybe a -> Maybe a -> Bool

thenCmp :: Ordering -> Ordering -> Ordering infixr 9

cmpList :: (a -> a -> Ordering) -> [a] -> [a] -> Ordering

removeSpaces :: String -> String

(<&&>) :: Applicative f => f Bool -> f Bool -> f Bool infixr 3

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

Edit distance

fuzzyMatch :: String -> [String] -> [String]

fuzzyLookup :: String -> [(String, a)] -> [a]

Search for possible matches to the users input in the given list, returning a small number of ranked results

Transitive closures

transitiveClosure :: (a -> [a]) -> (a -> a -> Bool) -> [a] -> [a]

Strictness

seqList :: [a] -> b -> b

Module names

looksLikeModuleName :: String -> Bool

looksLikePackageName :: String -> Bool

Argument processing

getCmd :: String -> Either String (String, String)

toCmdArgs :: String -> Either String (String, [String])

toArgs :: String -> Either String [String]

Integers

exactLog2 :: Integer -> Maybe Integer

Floating point

readRational :: String -> Rational

read helpers

maybeRead :: Read a => String -> Maybe a

maybeReadFuzzy :: Read a => String -> Maybe a

IO-ish utilities

doesDirNameExist :: FilePath -> IO Bool

getModificationUTCTime :: FilePath -> IO UTCTime

modificationTimeIfExists :: FilePath -> IO (Maybe UTCTime)

hSetTranslit :: Handle -> IO ()

global :: a -> IORef a

consIORef :: IORef [a] -> a -> IO ()

globalM :: IO a -> IORef a

sharedGlobal :: a -> (Ptr (IORef a) -> IO (Ptr (IORef a))) -> IORef a

sharedGlobalM :: IO a -> (Ptr (IORef a) -> IO (Ptr (IORef a))) -> IORef a

Filenames and paths

type Suffix = String

splitLongestPrefix :: String -> (Char -> Bool) -> (String, String)

escapeSpaces :: String -> String

data Direction

Constructors

Forwards

Backwards

reslash :: Direction -> FilePath -> FilePath

makeRelativeTo :: FilePath -> FilePath -> FilePath

Utils for defining Data instances

abstractConstr :: String -> Constr

abstractDataType :: String -> DataType

mkNoRepType :: String -> DataType

Constructs a non-representation for a non-representable type

Utils for printing C code

charToC :: Word8 -> String

Hashing

hashString :: String -> Int32

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.

Call stacks

data CallStack :: *

CallStacks are a lightweight method of obtaining a partial call-stack at any point in the program.

A function can request its call-site with the HasCallStack constraint. For example, we can define

errorWithCallStack :: HasCallStack => String -> a

as a variant of error that will get its call-site. We can access the call-stack inside errorWithCallStack with callStack.

errorWithCallStack :: HasCallStack => String -> a
errorWithCallStack msg = error (msg ++ "n" ++ prettyCallStack callStack)

Thus, if we call errorWithCallStack we will get a formatted call-stack alongside our error message.

>>> errorWithCallStack "die"
*** Exception: die
CallStack (from HasCallStack):
  errorWithCallStack, called at <interactive>:2:1 in interactive:Ghci1

GHC solves HasCallStack constraints in three steps:

1. If there is a CallStack in scope -- i.e. the enclosing function has a HasCallStack constraint -- GHC will append the new call-site to the existing CallStack.
2. If there is no CallStack in scope -- e.g. in the GHCi session above -- and the enclosing definition does not have an explicit type signature, GHC will infer a HasCallStack constraint for the enclosing definition (subject to the monomorphism restriction).
3. If there is no CallStack in scope and the enclosing definition has an explicit type signature, GHC will solve the HasCallStack constraint for the singleton CallStack containing just the current call-site.

CallStacks do not interact with the RTS and do not require compilation with -prof. On the other hand, as they are built up explicitly via the HasCallStack constraints, they will generally not contain as much information as the simulated call-stacks maintained by the RTS.

A CallStack is a [(String, SrcLoc)]. The String is the name of function that was called, the SrcLoc is the call-site. The list is ordered with the most recently called function at the head.

NOTE: The intrepid user may notice that HasCallStack is just an alias for an implicit parameter ?callStack :: CallStack. This is an implementation detail and should not be considered part of the CallStack API, we may decide to change the implementation in the future.

Since: 4.8.1.0

Instances

IsList CallStack

Be aware that 'fromList . toList = id' only for unfrozen CallStacks, since toList removes frozenness information.

Since: 4.9.0.0

Associated Types

type Item CallStack :: *

Methods

fromList :: [Item CallStack] -> CallStack

fromListN :: Int -> [Item CallStack] -> CallStack

toList :: CallStack -> [Item CallStack]

Show CallStack

Since: 4.9.0.0

Methods

showsPrec :: Int -> CallStack -> ShowS

show :: CallStack -> String

showList :: [CallStack] -> ShowS

NFData CallStack

Since: 1.4.2.0

Methods

rnf :: CallStack -> ()

type Item CallStack

type Item CallStack = (String, SrcLoc)

type HasCallStack = HasCallStack

A compatibility wrapper for the GHC.Stack.HasCallStack constraint.

type HasDebugCallStack = (() :: Constraint)

A call stack constraint, but only when isDebugOn.

prettyCurrentCallStack :: HasCallStack => String

Pretty-print the current callstack

Utils for flags

data OverridingBool

Constructors

Auto

Always

Never

Instances

Show OverridingBool

Methods

showsPrec :: Int -> OverridingBool -> ShowS

show :: OverridingBool -> String

showList :: [OverridingBool] -> ShowS

overrideWith :: Bool -> OverridingBool -> Bool