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Re: Need help from a newby
by ChrisK-5
::
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karle wrote:
>
>
> ChrisK-3 wrote:
>> karle wrote:
>>> My declaration is as followed:-
>>>
>>> type Address = Int
>>> data Port = C | D deriving(Eq,Show)
>>> data Payload = UP[Char] | RTDP(Address,Port) deriving(Eq,Show)
>>> data Pkgtype = RTD | U deriving(Eq,Show)
>>> type Pkg = (Pkgtype,Address,Payload)
>>> type Table = [(Address,Port)]
>>>
>>>
>>> findport::[Pkg]->[Table]->[([Pkg],[Pkg])]
>>> findport [(pt,ad,pa)] [(a,p)]
>>> | ( p == C) = ([pt,ad,a],[])
>>> | otherwise = ([],[pt,ad,a])
>>>
>>> Error received:-
>>>
>>> Type error in explicitly typed binding
>>> *** Term : [(a,p)]
>>> *** Type : [(a,b)]
>>> *** Does not match : [Table]
>>>
>>> Can anyone please help?
>> There seems to be several things that look wrong.
>>
>> Could you explain what findport is supposed to be doing?
>>
>> findport is just to check if the port in the Table is equal to C then it
>> will place the packet in the first of the list. Otherwise it will place
>> the packet in the second of the list.
So for each Pkg in [Pkg] the findport function takes the Address element in the
Pkg and looks it up in the Table. If the Address is C or D the original pkg
gets places in the first or second output list.
The 'partition' function from Data.List could help here:
http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-List.html#v%3Apartition
> partition :: (a -> Bool) -> [a] -> ([a], [a])
>
> The partition function takes a predicate a list and returns the pair of lists
> of elements which do and do not satisfy the predicate, ...
But your findport function is fairly broken right now.
The output (pt,ad,a) should be (pt,ad,pa). This was likely a typographical
error and is caught by the type system.
The declared type of the second argument should be Table, not [Table]. This is
the simple error that the compiler is reporting.
The first argument is matched against the pattern [(pt,ad,pa)] which only
matched lists with a single Pkg. This is also true of the second argument
getting pattern matched against [(a,p)] since this only matches Tables with a
single entry. These are errors in how findport has been written, I'll give you
a chance to try fixing each of these. You will need to handle lists with any
number of elements (including empty lists).
You also do not say what should happen to a Pkg with an Address that is not in
the Table.
Cheers,
Chris
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>
>
> ChrisK-3 wrote:
>> karle wrote:
>>> My declaration is as followed:-
>>>
>>> type Address = Int
>>> data Port = C | D deriving(Eq,Show)
>>> data Payload = UP[Char] | RTDP(Address,Port) deriving(Eq,Show)
>>> data Pkgtype = RTD | U deriving(Eq,Show)
>>> type Pkg = (Pkgtype,Address,Payload)
>>> type Table = [(Address,Port)]
>>>
>>>
>>> findport::[Pkg]->[Table]->[([Pkg],[Pkg])]
>>> findport [(pt,ad,pa)] [(a,p)]
>>> | ( p == C) = ([pt,ad,a],[])
>>> | otherwise = ([],[pt,ad,a])
>>>
>>> Error received:-
>>>
>>> Type error in explicitly typed binding
>>> *** Term : [(a,p)]
>>> *** Type : [(a,b)]
>>> *** Does not match : [Table]
>>>
>>> Can anyone please help?
>> There seems to be several things that look wrong.
>>
>> Could you explain what findport is supposed to be doing?
>>
>> findport is just to check if the port in the Table is equal to C then it
>> will place the packet in the first of the list. Otherwise it will place
>> the packet in the second of the list.
So for each Pkg in [Pkg] the findport function takes the Address element in the
Pkg and looks it up in the Table. If the Address is C or D the original pkg
gets places in the first or second output list.
The 'partition' function from Data.List could help here:
http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-List.html#v%3Apartition
> partition :: (a -> Bool) -> [a] -> ([a], [a])
>
> The partition function takes a predicate a list and returns the pair of lists
> of elements which do and do not satisfy the predicate, ...
But your findport function is fairly broken right now.
The output (pt,ad,a) should be (pt,ad,pa). This was likely a typographical
error and is caught by the type system.
The declared type of the second argument should be Table, not [Table]. This is
the simple error that the compiler is reporting.
The first argument is matched against the pattern [(pt,ad,pa)] which only
matched lists with a single Pkg. This is also true of the second argument
getting pattern matched against [(a,p)] since this only matches Tables with a
single entry. These are errors in how findport has been written, I'll give you
a chance to try fixing each of these. You will need to handle lists with any
number of elements (including empty lists).
You also do not say what should happen to a Pkg with an Address that is not in
the Table.
Cheers,
Chris
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@...
http://www.haskell.org/mailman/listinfo/haskell-cafe
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