Documentation

Loop with no arguments. This is the same as Fix.fix but the name is clearer.

Loop with a single state argument.

Loop with two state arguments. You could use loop1 with a pair, but sometimes the currying is convenient.

This is foldMap specialized to lists.

This is actually a mconcatMapM.

A further generalized version would be:

foldMapA :: (Applicative f, Traversable t, Monoid m) =>
   (a -> f m) -> t a -> f m
foldMapA f = fmap Foldable.fold . traverse f

Run the second action only if the first action returns Just.

This is like MaybeT, but using MaybeT itself required lots of annoying explicit lifting.

The Either equivalent of justm. EitherT solves the same problem, but requires a runEitherT and lots of hoistEithers.

Return the first action to return Just.

firstJust applied to a list.

Throw on Nothing.

I usually call this require.

I usually call this require_right.

A bifunctor is a type constructor that takes two type arguments and is a functor in both arguments. That is, unlike with Functor, a type constructor such as Either does not need to be partially applied for a Bifunctor instance, and the methods in this class permit mapping functions over the Left value or the Right value, or both at the same time.

Formally, the class Bifunctor represents a bifunctor from Hask -> Hask.

Intuitively it is a bifunctor where both the first and second arguments are covariant.

You can define a Bifunctor by either defining bimap or by defining both first and second.

If you supply bimap, you should ensure that:

bimap id id ≡ id

If you supply first and second, ensure:

first id ≡ id
second id ≡ id

If you supply both, you should also ensure:

bimap f g ≡ first f . second g

These ensure by parametricity:

bimap  (f . g) (h . i) ≡ bimap f h . bimap g i
first  (f . g) ≡ first  f . first  g
second (f . g) ≡ second f . second g

Since: base-4.8.0.0

Like find, but where the test can be monadic.

findM (Just . isUpper) "teST"             == Just (Just 'S')
findM (Just . isUpper) "test"             == Just Nothing
findM (Just . const True) ["x",undefined] == Just (Just "x")

A version of and lifted to a monad. Retains the short-circuiting behaviour.

andM [Just True,Just False,undefined] == Just False
andM [Just True,Just True ,undefined] == undefined
\xs -> Just (and xs) == andM (map Just xs)

A version of or lifted to a monad. Retains the short-circuiting behaviour.

orM [Just False,Just True ,undefined] == Just True
orM [Just False,Just False,undefined] == undefined
\xs -> Just (or xs) == orM (map Just xs)

A version of all lifted to a monad. Retains the short-circuiting behaviour.

allM Just [True,False,undefined] == Just False
allM Just [True,True ,undefined] == undefined
\(f :: Int -> Maybe Bool) xs -> anyM f xs == orM (map f xs)

A version of any lifted to a monad. Retains the short-circuiting behaviour.

anyM Just [False,True ,undefined] == Just True
anyM Just [False,False,undefined] == undefined
\(f :: Int -> Maybe Bool) xs -> anyM f xs == orM (map f xs)

Like not, but where the test can be monadic.

A version of mapMaybe that works with a monadic predicate.

A version of partition that works with a monadic predicate.

partitionM (Just . even) [1,2,3] == Just ([2], [1,3])
partitionM (const Nothing) [1,2,3] == Nothing

Like error, use when forced.

Throw an error in IO, with a stack.