haskell - Does the expressiveness of monads come at the expense of code reuse? -

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when compare binary operations of applicative , monad type class

(<*>) :: applicative f => f (a -> b) -> f -> f b (=<<) :: monad m       => (a -> m b) -> m -> m b

two differences become apparent:

  • ap expects normal, pure function, whereas bind expects monadic action, must return monad of same type
  • with ap sequence of actions determined applicative, whereas bind monadic action can determine control flow

so monads give me additional expressive power. however, since no longer accept normal, pure functions, expressiveness seems come @ expense of code reuse.

my conclusion might naive or false, since have merely little experience haskell , monadic computations. light in dark appreciated!

code reuse advantage if can reuse code want.

ghci> putstrln =<< getline sulfur sulfur ghci>

here, (=<<) picks string result produced in io context getline , feeds putstrln, prints said result.

ghci> :t getline getline :: io string ghci> :t putstrln putstrln :: string -> io () ghci> :t putstrln =<< getline putstrln =<< getline :: io ()

now, in type of fmap/(<$>)...

ghci> :t (<$>) (<$>) :: functor f => (a -> b) -> f -> f b

... possible b io (), , therefore nothing stops using putstrln it. however...

ghci> putstrln <$> getline sulfur ghci>

... nothing printed.

ghci> :t putstrln <$> getline putstrln <$> getline :: io (io ())

executing io (io ()) action won't execute inner io () action. that, need additional power of monad, either replacing (<$>) (=<<) or, equivalently, using join on io (io ()) value:

ghci> :t join join :: monad m => m (m a) -> m ghci> join (putstrln <$> getline) sulfur sulfur ghci>

like chi, have trouble understanding premises of question. seem expect 1 of functor, applicative , monad turn out better others. not case. can more things applicative functor, , more monad. if need additional power, use suitably powerful class. if not, using less powerful class lead simpler, clearer code , broader range of available instances.


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