ap package:rebase

In many situations, the liftM operations can be replaced by uses of ap, which promotes function application.

return f `ap` x1 `ap` ... `ap` xn

is equivalent to

liftMn f x1 x2 ... xn

This data type witnesses the lifting of a Monoid into an Applicative pointwise.

Recover the application operator <*> from select. Rigid selective functors satisfy the law <*> = apS and furthermore, the resulting applicative functor satisfies all laws of Applicative:

• Identity:

  pure id <*> v = v

• Homomorphism:

  pure f <*> pure x = pure (f x)

• Interchange:

  u <*> pure y = pure ($y) <*>
  u

• Composition:

  (.) <$> u <*> v <*> w = u
  <*> (v <*> w)

The computation appendFile file str function appends the string str, to the file file. Note that writeFile and appendFile write a literal string to a file. To write a value of any printable type, as with print, use the show function to convert the value to a string first.

main = appendFile "squares" (show [(x,x*x) | x <- [0,0.1..2]])

applyWhen applies a function to a value if a condition is true, otherwise, it returns the value unchanged. It is equivalent to flip (bool id). Algebraic properties:

• applyWhen True = id

• applyWhen False f = id

approxRational, applied to two real fractional numbers x and epsilon, returns the simplest rational number within epsilon of x. A rational number y is said to be simpler than another y' if

• abs (numerator y) <= abs (numerator y'), and
• denominator y <= denominator y'.

Any real interval contains a unique simplest rational; in particular, note that 0/1 is the simplest rational of all.

A functor with application, providing operations to

• embed pure expressions (pure), and
• sequence computations and combine their results (<*> and liftA2).

A minimal complete definition must include implementations of pure and of either <*> or liftA2. If it defines both, then they must behave the same as their default definitions:

(<*>) = liftA2 id

liftA2 f x y = f <$> x <*> y

Further, any definition must satisfy the following:

• Identity

  pure id <*> v =
  v

• Composition

  pure (.) <*> u
  <*> v <*> w = u <*> (v
  <*> w)

• Homomorphism

  pure f <*>
  pure x = pure (f x)

• Interchange

  u <*> pure y =
  pure ($ y) <*> u

The other methods have the following default definitions, which may be overridden with equivalent specialized implementations:

• u *> v = (id <$ u)
  <*> v

• u <* v = liftA2 const u v

As a consequence of these laws, the Functor instance for f will satisfy

• fmap f x = pure f <*> x

It may be useful to note that supposing

forall x y. p (q x y) = f x . g y

it follows from the above that

liftA2 p (liftA2 q u v) = liftA2 f u . liftA2 g v

If f is also a Monad, it should satisfy

• pure = return

• m1 <*> m2 = m1 >>= (\x1 -> m2
  >>= (\x2 -> return (x1 x2)))

• (*>) = (>>)

(which implies that pure and <*> satisfy the applicative functor laws).

A strong lax semi-monoidal endofunctor. This is equivalent to an Applicative without pure. Laws:

(.) <$> u <.> v <.> w = u <.> (v <.> w)
x <.> (f <$> y) = (. f) <$> x <.> y
f <$> (x <.> y) = (f .) <$> x <.> y

The laws imply that .> and <. really ignore their left and right results, respectively, and really return their right and left results, respectively. Specifically,

(mf <$> m) .> (nf <$> n) = nf <$> (m .> n)
(mf <$> m) <. (nf <$> n) = mf <$> (m <. n)

A more meaningful and conflict-free alias for first.

A more meaningful and conflict-free alias for second.

Some arrows allow application of arrow inputs to other inputs. Instances should satisfy the following laws:

• first (arr (\x -> arr (\y ->
  (x,y)))) >>> app = id

• first (arr (g >>>)) >>>
  app = second g >>> app

• first (arr (>>> h)) >>>
  app = app >>> h

Such arrows are equivalent to monads (see ArrowMonad).