What is the difference between dot and dollar sign

Question

What is the difference between the dot     and the dollar sign       As I understand it  they are both syntactic sugar for not needing to use parentheses

User · Answer

One application that is useful and took me some time to figure out from the very short description at learn you a haskell: Since:

f $ x = f x

and parenthesizing the right hand side of an expression containing an infix operator converts it to a prefix function, one can write ($ 3) (4+) analogous to (++", world") "hello".

Why would anyone do this? For lists of functions, for example. Both:

map (++", world") ["hello","goodbye"]`

and:

map ($ 3) [(4+),(3*)]

are shorter than map (\x -> x ++ ", world") ... or map (\f -> f 3) .... Obviously, the latter variants would be more readable for most people.

User · Answer

Haskell  difference between    dot  and    dollar sign  What is the difference between the dot     and the dollar sign       As I understand it  they are both syntactic sugar for not needing to use parentheses   They are not syntactic sugar for not needing to use parentheses - they are functions  - infixed  thus we may call them operators  Compose       and when to use it      is the compose function  So result    f   g  x  is the same as building a function that passes the result of its argument passed to g on to f  h    x - gt  f  g x  result   h x  Use     when you don t have the arguments available to pass to the functions you wish to compose  Right associative apply       and when to use it     is a right-associative apply function with low binding precedence  So it merely calculates the things to the right of it first  Thus  result   f   g x  is the same as this  procedurally  which matters since Haskell is evaluated lazily  it will begin to evaluate f first   h   f g x   g x result   h g x  or more concisely  result   f  g x   Use     when you have all the variables to evaluate before you apply the preceding function to the result  We can see this by reading the source for each function  Read the Source Here s the source for      --   Function composition   -  INLINE      -  -- Make sure it has TWO args only on the left  so that it inlines -- when applied to two functions  even if there is no final argument            b - gt  c  - gt   a - gt  b  - gt  a - gt  c     f g    x - gt  f  g x   And here s the source for      --   Application operator   This operator is redundant  since ordinary -- application   f x   means the same as   f     x    However      has -- low  right-associative binding precedence  so it sometimes allows -- parentheses to be omitted  for example  -- --  gt      f   g   h x     f  g  h x   -- -- It is also useful in higher-order situations  such as   map       0  xs   -- or   Data List zipWith        fs xs    -  INLINE      -                              a - gt  b  - gt  a - gt  b f   x                      f x  Conclusion Use composition when you do not need to immediately evaluate the function  Maybe you want to pass the function that results from composition to another function  Use application when you are supplying all arguments for full evaluation  So for our example  it would be semantically preferable to do f   g x  when we have x  or rather  g s arguments   and do  f   g  when we don t

User · Answer

Also note that     is the identity function specialised to function types  The identity function looks like this   id    a - gt  a id x   x   While     looks like this           a - gt  b  - gt   a - gt  b        id   Note that I ve intentionally added extra parentheses in the type signature   Uses of     can usually be eliminated by adding parenthesis  unless the operator is used in a section   E g   f   g x becomes f  g x     Uses of     are often slightly harder to replace  they usually need a lambda or the introduction of an explicit function parameter  For example   f   g   h   becomes  f x    g   h  x   becomes  f x   g  h x    Hope this helps

User · Answer

or you could avoid the   and   constructions by using pipelining   third xs   xs   gt  tail   gt  tail   gt  head   That s after you ve added in the helper function      gt   x y   y x

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allows functions to be chained together without adding parentheses to control evaluation order   Prelude gt  head  tail  asdf    s   Prelude gt  head   tail  asdf   s    The compose operator     creates a new function without specifying the arguments   Prelude gt  let second x   head   tail x Prelude gt  second  asdf   s   Prelude gt  let second   head   tail Prelude gt  second  asdf   s    The example above is arguably illustrative  but doesn t really show the convenience of using composition   Here s another analogy   Prelude gt  let third x   head   tail   tail x Prelude gt  map third   asdf    qwer    1234    de3    If we only use third once  we can avoid naming it by using a lambda   Prelude gt  map   x - gt  head   tail   tail x    asdf    qwer    1234    de3    Finally  composition lets us avoid the lambda   Prelude gt  map  head   tail   tail    asdf    qwer    1234    de3

User · Answer

The   operator is for avoiding parentheses   Anything appearing after it will take precedence over anything that comes before   For example  let s say you ve got a line that reads   putStrLn  show  1   1     If you want to get rid of those parentheses  any of the following lines would also do the same thing   putStrLn  show   1   1  putStrLn   show  1   1  putStrLn   show   1   1   The primary purpose of the   operator is not to avoid parentheses  but to chain functions  It lets you tie the output of whatever appears on the right to the input of whatever appears on the left   This usually also results in fewer parentheses  but works differently   Going back to the same example   putStrLn  show  1   1       1   1  doesn t have an input  and therefore cannot be used with the   operator  show can take an Int and return a String  putStrLn can take a String and return an IO       You can chain show to putStrLn like this    putStrLn   show   1   1    If that s too many parentheses for your liking  get rid of them with the   operator   putStrLn   show   1   1

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A great way to learn more about anything  any function  is to remember that everything is a function  That general mantra helps  but in specific cases like operators  it helps to remember this little trick    t             b - gt  c  - gt   a - gt  b  - gt  a - gt  c   and   t             a - gt  b  - gt  a - gt  b   Just remember to use  t liberally  and wrap your operators in

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All the other answers are pretty good  But there   s an important usability detail about how ghc treats    that the ghc type checker allows for instatiarion with higher rank  quantified types   If you look at the type of    id for example you   ll find it   s gonna take a function whose argument is itself a polymorphic function  Little things like that aren   t given the same flexibility with an equivalent upset operator    This actually makes me wonder if    deserves the same treatment or not

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The most important part about   is that it has the lowest operator precedence   If you type info you ll see this     gt   info             a - gt  b  - gt  a - gt  b     -- Defined in    GHC Base    infixr 0     This tells us it is an infix operator with right-associativity that has the lowest possible precedence  Normal function application is left-associative and has highest precedence  10   So   is something of the opposite    So then we use it where normal function application or using    doesn t work    So  for example  this works     gt  head   sort    example    gt  e   but this does not     gt  head   sort  example    because   has lower precedence than sort and the type of  sort  example   is  Char      gt   type  sort  example    sort  example       Char    But   expects two functions and there isn t a nice short way to do this because of the order of operations of sort and

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The short and sweet version        calls the function which is its left-hand argument on the value which is its right-hand argument      composes the function which is its left-hand argument on the function which is its right-hand argument

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My rule is simple  I m beginner too      do not use   if you want to pass the parameter  call the function   and  do not use   if there is no parameter yet  compose a function    That is   show   head  1  2    but never   show   head  1  2

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They have different types and different definitions   infixr 9           b - gt  c  - gt   a - gt  b  - gt   a - gt  c   f   g  x   f  g x   infixr 0           a - gt  b  - gt  a - gt  b f   x   f x       is intended to replace normal function application but at a different precedence to help avoid parentheses      is for composing two functions together to make a new function   In some cases they are interchangeable  but this is not true in general  The typical example where they are is   f   g   h   x        f   g   h   x   In other words in a chain of  s  all but the final one can be replaced by

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I think a short example of where you would use   and not   would help clarify things   double x   x   2 triple x   x   3 times6   double   triple   i times6 times6    Num c   gt  c - gt  c   Note that times6 is a function that is created from function composition

[haskell] What is the difference between . (dot) and $ (dollar sign)?

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