filterFirst

[Alexteslin]

Hi,
first I like to thank all of you guys - it really helps!

I am still on a same chapter with higher order functions and this function is also confusing.
But before i even define this function i am getting the type error - i don't know why? So i wrote the simpler one like:

filterAlpha :: (a -> Bool) -> [a] -> [a]
filterAlpha f [] = []
filterAlpha f (x:xs)
        |f x = x : filterAlpha xs
        |otherwise = filterAlpha xs


and i am getting this error message:

Type error in application
Expression :filterAlpha xs
Type        : [b]
Dous not match : a -> Bool

To even my very little knowledge i think that this should work. What am i doing wrong?

Thanks again  

[Heinrich Apfelmus]

Alexteslin wrote:

> filterAlpha :: (a -> Bool) -> [a] -> [a]
> filterAlpha f [] = []
> filterAlpha f (x:xs)
> |f x = x : filterAlpha xs
> |otherwise = filterAlpha xs
>
>
> and i am getting this error message:
>
> Type error in application
> Expression :filterAlpha xs
> Type        : [b]
> Dous not match : a -> Bool

  filterAlpha :: (a -> Bool) -> [a] -> [a]
  filterAlpha f [] = []
  filterAlpha f (x:xs)
     | f x       = x : filterAlpha f xs
     | otherwise =     filterAlpha f xs

filterAlpha  has two parameters. The first parameter is a function (a ->
Bool), the second is a list [a]. The error message complains that  xs ,
which you actidentially gave as first parameter, is a list [a] and not a
function (a -> Bool).

Regards,
apfelmus

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[Jonathan Cast]

On Monday 23 July 2007, Alexteslin wrote:

> Hi,
> first I like to thank all of you guys - it really helps!
>
> I am still on a same chapter with higher order functions and this function
> is also confusing.
> But before i even define this function i am getting the type error - i
> don't know why? So i wrote the simpler one like:
>
> filterAlpha :: (a -> Bool) -> [a] -> [a]
> filterAlpha f [] = []
> filterAlpha f (x:xs)
> |f x = x : filterAlpha xs
> |otherwise = filterAlpha xs
>
> and i am getting this error message:
>
> Type error in application
> Expression :filterAlpha xs
> Type        : [b]
> Dous not match : a -> Bool
>
> To even my very little knowledge i think that this should work. What am i
> doing wrong?

You're passing only one argument to filterApha, when it takes two.  Haskell
can't figure out which arguments you want to stay the same on every recursive
call, and which ones you want to vary, so you have to supply every argument
every time.

Jonathan Cast
http://sourceforge.net/projects/fid-core
http://sourceforge.net/projects/fid-emacs
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[Alexteslin]

apfelmus wrote

Alexteslin wrote:
> filterAlpha :: (a -> Bool) -> [a] -> [a]
> filterAlpha f [] = []
> filterAlpha f (x:xs)
> |f x = x : filterAlpha xs
> |otherwise = filterAlpha xs
>
>
> and i am getting this error message:
>
> Type error in application
> Expression :filterAlpha xs
> Type        : [b]
> Dous not match : a -> Bool

  filterAlpha :: (a -> Bool) -> [a] -> [a]
  filterAlpha f [] = []
  filterAlpha f (x:xs)
     | f x       = x : filterAlpha f xs
     | otherwise =     filterAlpha f xs

filterAlpha  has two parameters. The first parameter is a function (a ->
Bool), the second is a list [a]. The error message complains that  xs ,
which you actidentially gave as first parameter, is a list [a] and not a
function (a -> Bool).

Regards,
apfelmus

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Oh silly me.  I defined firstFirst now, thank you.

[Dan Weston]

If you find it tedious to pass parameters that never change, remember
that you can access symbols defined in the enclosing environment
(closure), freeing you from "passing it on" each time:

filterAlpha :: (a -> Bool) -> [a] -> [a]
filterAlpha f = filterAlpha'
   where filterAlpha' [] = []
         filterAlpha' (x:xs)
           | f x         = x : (filterAlpha' xs)
           | otherwise   =     (filterAlpha' xs)

As far as the primed version filterAlpha' is concerned, f is a global
symbol. The argument list is just used for values that vary.

This will be your friend if the parameter list starts to stack up with
more and more "reference" or "environment" inputs. It is also easier to
see that f never changes if it is defined in only one spot.

Later, when you study monads you will notice the same pattern in the
Reader monad, where the technique is even more valuable.

Dan Weston

Alexteslin wrote:
>
> filterAlpha :: (a -> Bool) -> [a] -> [a]
> filterAlpha f [] = []
> filterAlpha f (x:xs)
> |f x   = x : filterAlpha f xs    -- corrected
> |otherwise = filterAlpha f xs    -- corrected


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[Alexteslin]

Thank you Dan

Dan Weston wrote

If you find it tedious to pass parameters that never change, remember
that you can access symbols defined in the enclosing environment
(closure), freeing you from "passing it on" each time:

filterAlpha :: (a -> Bool) -> [a] -> [a]
filterAlpha f = filterAlpha'
   where filterAlpha' [] = []
         filterAlpha' (x:xs)
           | f x         = x : (filterAlpha' xs)
           | otherwise   =     (filterAlpha' xs)

As far as the primed version filterAlpha' is concerned, f is a global
symbol. The argument list is just used for values that vary.

This will be your friend if the parameter list starts to stack up with
more and more "reference" or "environment" inputs. It is also easier to
see that f never changes if it is defined in only one spot.

Later, when you study monads you will notice the same pattern in the
Reader monad, where the technique is even more valuable.

Dan Weston

Alexteslin wrote:
>
> filterAlpha :: (a -> Bool) -> [a] -> [a]
> filterAlpha f [] = []
> filterAlpha f (x:xs)
> |f x   = x : filterAlpha f xs    -- corrected
> |otherwise = filterAlpha f xs    -- corrected


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