Prelude> :i ShowS
type ShowS = String > String  Defined in `GHC.Show' It is a type of a function? I cannot understand this type, and don't know how to create functions of this type. And this function "shows" Prelude> :i shows shows :: Show a => a > ShowS  Defined in `GHC.Show' I don't know how this function works. Yi  next part  An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130924/8266243a/attachment.htm> 
Looks like it's a convenience for building up a compositions of "Show"s.
ShowS is indeed a synonym for a function. The type of shows alone isn't enough to figure out how it behaves exactly, but testing it out in GHCi is telling: > [Prelude] ? :i ShowS > type ShowS = String > String  Defined in `GHC.Show' > [Prelude] ? :i shows > shows :: Show a => a > ShowS  Defined in `GHC.Show' > [Prelude] ? shows "asdf" "qwer" > "\"asdf\"qwer" On Tue, Sep 24, 2013 at 8:15 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: > Prelude> :i ShowS > type ShowS = String > String  Defined in `GHC.Show' > > It is a type of a function? I cannot understand this type, and don't know > how to create functions of this type. > > And this function "shows" > > Prelude> :i shows > shows :: Show a => a > ShowS  Defined in `GHC.Show' > > I don't know how this function works. > > Yi > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130924/25df9a6f/attachment.htm> 
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In reply to this post by yi lu
On Tue, Sep 24, 2013 at 5:15 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote:
> Prelude> :i ShowS > type ShowS = String > String  Defined in `GHC.Show' > > It is a type of a function? I cannot understand this type, and don't know > how to create functions of this type. When you say "cannot" you probably mean "do not". Among the reasons are this lack of ability is highly transient. And this transience is highly dependent on the steps you take to change this. There are heaps of freely available intro material on this. What have you looked at?  KimEe  next part  An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130924/d06d2c5c/attachment.htm> 
In reply to this post by Lyndon Maydwell
On Tue, Sep 24, 2013 at 6:54 PM, Lyndon Maydwell <maydwell at gmail.com> wrote:
> Looks like it's a convenience for building up a compositions of "Show"s. > > ShowS is indeed a synonym for a function. The type of shows alone isn't > enough to figure out how it behaves exactly, but testing it out in GHCi is > telling: > > > > [Prelude] ? :i ShowS > > type ShowS = String > String  Defined in `GHC.Show' > > [Prelude] ? :i shows > > shows :: Show a => a > ShowS  Defined in `GHC.Show' > > [Prelude] ? shows "asdf" "qwer" > > "\"asdf\"qwer" > > I don't know the meaning of this result. > > On Tue, Sep 24, 2013 at 8:15 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: > >> Prelude> :i ShowS >> type ShowS = String > String  Defined in `GHC.Show' >> >> It is a type of a function? I cannot understand this type, and don't know >> how to create functions of this type. >> >> And this function "shows" >> >> Prelude> :i shows >> shows :: Show a => a > ShowS  Defined in `GHC.Show' >> >> I don't know how this function works. >> >> Yi >> >> _______________________________________________ >> Beginners mailing list >> Beginners at haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> >> > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130924/9072ba7d/attachment.htm> 
In reply to this post by KimEe Yeoh
I have just looked at the API of Prelude, and I remember similar definition
for parallel haskell. On Tue, Sep 24, 2013 at 7:17 PM, KimEe Yeoh <ky3 at atamo.com> wrote: > > On Tue, Sep 24, 2013 at 5:15 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: > >> Prelude> :i ShowS >> type ShowS = String > String  Defined in `GHC.Show' >> >> It is a type of a function? I cannot understand this type, and don't know >> how to create functions of this type. > > > When you say "cannot" you probably mean "do not". Among the reasons are > this lack of ability is highly transient. And this transience is highly > dependent on the steps you take to change this. > > There are heaps of freely available intro material on this. What have you > looked at? > >  KimEe > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130924/cf81c372/attachment.htm> 
In reply to this post by yi lu
What does `show "asdf"` give you in ghci? What Lyndon showed you was that
this function is equivalent to `shows x s = show x ++ s`. Have fun, Shrivats On Sep 24, 2013 5:09 PM, "yi lu" <zhiwudazhanjiangshi at gmail.com> wrote: > > On Tue, Sep 24, 2013 at 6:54 PM, Lyndon Maydwell <maydwell at gmail.com>wrote: > >> Looks like it's a convenience for building up a compositions of "Show"s. >> >> ShowS is indeed a synonym for a function. The type of shows alone isn't >> enough to figure out how it behaves exactly, but testing it out in GHCi is >> telling: >> >> >> > [Prelude] ? :i ShowS >> > type ShowS = String > String  Defined in `GHC.Show' >> > [Prelude] ? :i shows >> > shows :: Show a => a > ShowS  Defined in `GHC.Show' >> > [Prelude] ? shows "asdf" "qwer" >> > "\"asdf\"qwer" >> >> I don't know the meaning of this result. > > > >> >> On Tue, Sep 24, 2013 at 8:15 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: >> >>> Prelude> :i ShowS >>> type ShowS = String > String  Defined in `GHC.Show' >>> >>> It is a type of a function? I cannot understand this type, and don't >>> know how to create functions of this type. >>> >>> And this function "shows" >>> >>> Prelude> :i shows >>> shows :: Show a => a > ShowS  Defined in `GHC.Show' >>> >>> I don't know how this function works. >>> >>> Yi >>> >>> _______________________________________________ >>> Beginners mailing list >>> Beginners at haskell.org >>> http://www.haskell.org/mailman/listinfo/beginners >>> >>> >> >> _______________________________________________ >> Beginners mailing list >> Beginners at haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> >> > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130924/31b4e03a/attachment.htm> 
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In reply to this post by yi lu
On Tue, Sep 24, 2013 at 6:43 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote:
> I have just looked at the API of Prelude, and I remember similar > definition for parallel haskell. How far have you gotten with LYAH? Or Hutton's textbook? What does a search on "haskell intro type system" reveal?  KimEe  next part  An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130924/dd51afb3/attachment.htm> 
In reply to this post by Shrivats
On Tue, Sep 24, 2013 at 7:47 PM, Shrivats <shrvtsnvs at gmail.com> wrote:
> What does `show "asdf"` give you in ghci? What Lyndon showed you was that > this function is equivalent to `shows x s = show x ++ s`. > > Ah, it works a bit like "++" ? > Have fun, > > Shrivats > On Sep 24, 2013 5:09 PM, "yi lu" <zhiwudazhanjiangshi at gmail.com> wrote: > >> >> On Tue, Sep 24, 2013 at 6:54 PM, Lyndon Maydwell <maydwell at gmail.com>wrote: >> >>> Looks like it's a convenience for building up a compositions of "Show"s. >>> >>> ShowS is indeed a synonym for a function. The type of shows alone isn't >>> enough to figure out how it behaves exactly, but testing it out in GHCi is >>> telling: >>> >>> >>> > [Prelude] ? :i ShowS >>> > type ShowS = String > String  Defined in `GHC.Show' >>> > [Prelude] ? :i shows >>> > shows :: Show a => a > ShowS  Defined in `GHC.Show' >>> > [Prelude] ? shows "asdf" "qwer" >>> > "\"asdf\"qwer" >>> >>> I don't know the meaning of this result. >> >> >> >>> >>> On Tue, Sep 24, 2013 at 8:15 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: >>> >>>> Prelude> :i ShowS >>>> type ShowS = String > String  Defined in `GHC.Show' >>>> >>>> It is a type of a function? I cannot understand this type, and don't >>>> know how to create functions of this type. >>>> >>>> And this function "shows" >>>> >>>> Prelude> :i shows >>>> shows :: Show a => a > ShowS  Defined in `GHC.Show' >>>> >>>> I don't know how this function works. >>>> >>>> Yi >>>> >>>> _______________________________________________ >>>> Beginners mailing list >>>> Beginners at haskell.org >>>> http://www.haskell.org/mailman/listinfo/beginners >>>> >>>> >>> >>> _______________________________________________ >>> Beginners mailing list >>> Beginners at haskell.org >>> http://www.haskell.org/mailman/listinfo/beginners >>> >>> >> >> _______________________________________________ >> Beginners mailing list >> Beginners at haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> >> > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130924/0ae01f1c/attachment.htm> 
I have found the link of difference list. Maybe this helps.
http://www.haskell.org/haskellwiki/Difference_list On Tue, Sep 24, 2013 at 7:54 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: > > > > On Tue, Sep 24, 2013 at 7:47 PM, Shrivats <shrvtsnvs at gmail.com> wrote: > >> What does `show "asdf"` give you in ghci? What Lyndon showed you was that >> this function is equivalent to `shows x s = show x ++ s`. >> >> Ah, it works a bit like "++" ? > >> Have fun, >> >> Shrivats >> On Sep 24, 2013 5:09 PM, "yi lu" <zhiwudazhanjiangshi at gmail.com> wrote: >> >>> >>> On Tue, Sep 24, 2013 at 6:54 PM, Lyndon Maydwell <maydwell at gmail.com>wrote: >>> >>>> Looks like it's a convenience for building up a compositions of "Show"s. >>>> >>>> ShowS is indeed a synonym for a function. The type of shows alone isn't >>>> enough to figure out how it behaves exactly, but testing it out in GHCi is >>>> telling: >>>> >>>> >>>> > [Prelude] ? :i ShowS >>>> > type ShowS = String > String  Defined in `GHC.Show' >>>> > [Prelude] ? :i shows >>>> > shows :: Show a => a > ShowS  Defined in `GHC.Show' >>>> > [Prelude] ? shows "asdf" "qwer" >>>> > "\"asdf\"qwer" >>>> >>>> I don't know the meaning of this result. >>> >>> >>> >>>> >>>> On Tue, Sep 24, 2013 at 8:15 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: >>>> >>>>> Prelude> :i ShowS >>>>> type ShowS = String > String  Defined in `GHC.Show' >>>>> >>>>> It is a type of a function? I cannot understand this type, and don't >>>>> know how to create functions of this type. >>>>> >>>>> And this function "shows" >>>>> >>>>> Prelude> :i shows >>>>> shows :: Show a => a > ShowS  Defined in `GHC.Show' >>>>> >>>>> I don't know how this function works. >>>>> >>>>> Yi >>>>> >>>>> _______________________________________________ >>>>> Beginners mailing list >>>>> Beginners at haskell.org >>>>> http://www.haskell.org/mailman/listinfo/beginners >>>>> >>>>> >>>> >>>> _______________________________________________ >>>> Beginners mailing list >>>> Beginners at haskell.org >>>> http://www.haskell.org/mailman/listinfo/beginners >>>> >>>> >>> >>> _______________________________________________ >>> Beginners mailing list >>> Beginners at haskell.org >>> http://www.haskell.org/mailman/listinfo/beginners >>> >>> >> _______________________________________________ >> Beginners mailing list >> Beginners at haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> >> > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130924/8d976e9b/attachment.htm> 
In reply to this post by KimEe Yeoh
On Tue, Sep 24, 2013 at 7:50 PM, KimEe Yeoh <ky3 at atamo.com> wrote:
> > On Tue, Sep 24, 2013 at 6:43 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: > >> I have just looked at the API of Prelude, and I remember similar >> definition for parallel haskell. > > > How far have you gotten with LYAH? Or Hutton's textbook? > > I don't know this problem is revealed in LYAH, and I will check it now. > What does a search on "haskell intro type system" reveal? > >  KimEe > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130924/b197571c/attachment.htm> 
In reply to this post by yi lu
ShowS is just a type alias for String > String. Anywhere where you could
put a function of type String > String you could replace that with ShowS. Examples blah :: String > String blah "abc" = "def" is no different than blah :: Shows blah "abc" = "def" >From the GHC.Show import GHC.Show.showList__ :: (a > ShowS) > [a] > ShowS is equivalent to GHC.Show.showList__ :: (a > (String > String)) > [a] > (String > String) It can be a little confusing but a lot of times you use the same function prototype and it is useful to just turn it into its own little type to shorten the types in your code. On Tue, Sep 24, 2013 at 6:15 AM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: > Prelude> :i ShowS > type ShowS = String > String  Defined in `GHC.Show' > > It is a type of a function? I cannot understand this type, and don't know > how to create functions of this type. > > And this function "shows" > > Prelude> :i shows > shows :: Show a => a > ShowS  Defined in `GHC.Show' > > I don't know how this function works. > > Yi > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130924/38cf4446/attachment.htm> 
In reply to this post by yi lu
to @KimEe
I find it here. Thanks again. http://learnyouahaskell.com/forafewmonadsmore On Tue, Sep 24, 2013 at 8:36 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: > > > > On Tue, Sep 24, 2013 at 7:50 PM, KimEe Yeoh <ky3 at atamo.com> wrote: > >> >> On Tue, Sep 24, 2013 at 6:43 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: >> >>> I have just looked at the API of Prelude, and I remember similar >>> definition for parallel haskell. >> >> >> How far have you gotten with LYAH? Or Hutton's textbook? >> >> I don't know this problem is revealed in LYAH, and I will check it now. > Thanks. > > >> What does a search on "haskell intro type system" reveal? >> >>  KimEe >> >> _______________________________________________ >> Beginners mailing list >> Beginners at haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> >> > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130925/7045f65d/attachment.htm> 
Although the links (difflist, forafewmonadsmore) that you posted are
very interesting in their own right, you only need to understand the following in order to comprehend "shows" and "ShowS"... * TypeClasses * Type Synonyms * Partial Application * Function Composition Type Classes ========== There are many references for understanding how these work, but the "shows" function simply depends on its first parameter being able to be shown through the use of the "Show" typeclass. Type Synonyms ============ Any type can have an "alias" created in the form of a typesynonym. Although these can be parameterised, in the case of "ShowS" it is not: `type ShowS = String > String` This means that wherever you see a reference to "ShowS" in a type, you may replace it with "String > String". Partial application ============= Look at a reference implementation of "shows" that Shrivats has described: `shows x s = show x ++ s` The type signature of "shows" focuses on the partially applied viewpoint, because (thanks to the typesynonym) it is written as if it only takes one argument: `shows :: Show a => a > ShowS` However, with the "ShowS" synonym resolved, you can see that it actually takes two: `shows :: Show a => a > String > String` Keep in mind that although Shrivats implementation is semantically equivalent to the one in GHC.List, the performance characteristics may be different. Function Composition ================ Although this isn't strictly required in order to understand "shows", it provides a potential motivation for why the function exists. If you wished to chain together a bunch of String representations of various objects, leaving the possibility of adding more later, you would have to use many lambdas if you wished to constrain yourself to using "show" and "++". For example: `showWithMore a = \x > show a ++ x` Applying these would become tedious: `myBools = \x > showWithMore True (showWithMore False x)` Thankfully, this can be avoided through the use of functioncomposition: ``myBools = shows True . shows False` Hopefully this goes some way to getting you to the core of the construction and motivation of these functions and synonyms. As always, for a true understanding of the motivations behind such a function, nothing beats looking at the source code [1]. [1]  http://hackage.haskell.org/packages/archive/base/3.0.3.2/doc/html/src/GHCShow.html#ShowS On Wed, Sep 25, 2013 at 10:15 AM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: > to @KimEe > > I find it here. Thanks again. > http://learnyouahaskell.com/forafewmonadsmore > > > > On Tue, Sep 24, 2013 at 8:36 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: > >> >> >> >> On Tue, Sep 24, 2013 at 7:50 PM, KimEe Yeoh <ky3 at atamo.com> wrote: >> >>> >>> On Tue, Sep 24, 2013 at 6:43 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote: >>> >>>> I have just looked at the API of Prelude, and I remember similar >>>> definition for parallel haskell. >>> >>> >>> How far have you gotten with LYAH? Or Hutton's textbook? >>> >>> I don't know this problem is revealed in LYAH, and I will check it now. >> Thanks. >> >> >>> What does a search on "haskell intro type system" reveal? >>> >>>  KimEe >>> >>> _______________________________________________ >>> Beginners mailing list >>> Beginners at haskell.org >>> http://www.haskell.org/mailman/listinfo/beginners >>> >>> >> > > _______________________________________________ > Beginners mailing list > Beginners at haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130925/aef01171/attachment.htm> 
In reply to this post by yi lu
On Tue, Sep 24, 2013 at 3:45 PM, yi lu <zhiwudazhanjiangshi at gmail.com>wrote:
> Prelude> :i ShowS > type ShowS = String > String  Defined in `GHC.Show' > > It is a type of a function? I cannot understand this type, and don't know > how to create functions of this type. > > And this function "shows" > > Prelude> :i shows > shows :: Show a => a > ShowS  Defined in `GHC.Show' > > I don't know how this function works. > > Look at the picture at start of http://en.wikipedia.org/wiki/Linked_list which corresponds to the Haskell list a = [12,19,37] and 'a' would be a pointer pointing to the 12node. Now if we wanted to add something  say 8  before 12, its a simple operation: Make a new node containing 8, point it to the 12node Assign a to this new node [Remember we are in imperativeland!!] This is a simple or constanttime operation However adding something to the end is harder: we have to start from a and walk down the list till the end and then mutate it to the new 8node  an O(n) operation where n is the length of the list. How can we have an O(1) add_to_end operation? Well in imperative land there are many approaches, one of which is that for every list like a, we also store an a_end pointing to the last element of a and then mutate that. This converts an O(n) operation into an O(1) operation at the cost of a mere extra pointer. shows is the same idea in FPland! The haskell list a = [12,19,37] is as you surely know a = 12 : (19 : (37: [])) Replace the [] all the way in with an x and to make it valid lambdabind it; ie a = \ x > 12 : (19 : (37 : x)) a :: [Int] > [Int] If the Int were Char that would be the Shows type And that lambdaexp basically constitutes your 'pointertotheend' In particular with normal lists [12,19,37] ++ some_list needs 3 steps to walk down the first list However instead of [12,19,37] if we have the lambdaexp 'a' above we just need to do a some_list and we have append being done by a single beta reduction step! Voila!  next part  An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20130925/403a52e7/attachment.htm> 
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