Network.CGI -- practical web programming example.

[Edward Ing-2]

I am somewhat new to haskell. It is amazing that I can actually write
a CGI program using Network.CGI without really being comfortable with
the Haskell type system. Especially when it involves monad
transformations.
So I decided that I better understand this. I looked at the Practical
Web Programming examples to try to understand what is going on. I came
up with a problem that might demonstrate my misunderstanding. I am
wondering if you can answer questions I have. Code 1 is the example
from PWP, Code 2 is my variation and it works so I am stumped by what
the liftM is required.

code 1>>>
#!/usr/bin/runghc
import Network.CGI
import Text.XHtml

import qualified Data.ByteString.Lazy as BS

import Control.Monad (liftM)
import Data.Maybe (fromJust)

uploadDir = "../upload"

fileForm = form ! [method "post", enctype "multipart/form-data"]
            << [afile "file", submit "" "Upload"]
saveFile n =
   do          cont <- (liftM fromJust) $ getInputFPS "file"
        let f = uploadDir ++ "/" ++ basename n
        liftIO $ BS.writeFile f cont
        return $ paragraph << ("Saved as " +++ anchor ! [href f] << f +++ ".")

page t b = header << thetitle << t +++ body << b

basename = reverse . takeWhile (`notElem` "/\\") . reverse

cgiMain =
   do mn <- getInputFilename "file"
      h <- maybe (return fileForm) saveFile mn
      output $ renderHtml $ page "Upload example" h

main = runCGI $ handleErrors cgiMain

Code 2 (modifier) >>>

import Network.CGI
import Text.XHtml

import qualified Data.ByteString.Lazy as BS

import Control.Monad (liftM)
import Data.Maybe (fromJust)

uploadDir = "../upload"

fileForm = form ! [method "post", enctype "multipart/form-data"]
            << [afile "file", submit "" "Upload"]
saveFile n =
do   cont <- getInputFPS "file"
       let f = uploadDir ++ "/" ++ basename n
      liftIO $ BS.writeFile f (fromJust cont)
      return $ paragraph << ("Saved as " +++ anchor ! [href f] << f +++ ".")

page t b = header << thetitle << t +++ body << b

basename = reverse . takeWhile (`notElem` "/\\") . reverse

cgiMain =
   do mn <- getInputFilename "file"
      h <- maybe (return fileForm) saveFile mn
      output $ renderHtml $ page "Upload example" h

main = runCGI $ handleErrors cgiMain



Questions ===

1) Why did the author choose to insert "liftM" in function saveFile?
It doesn't seem necessary in my version.

2) My background mainly is Java but here is my understanding of Monad
Transforms. The CGIT m type carries around with it the CGI Request
context and response contexts. The transformations (lifts) is similar
to casting so that you can use the functions for specific
manifestations but it also encapsulates the data. Is this correct?


Edward
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[Luke Palmer-2]

Your code examples are:

On Sat, Jun 27, 2009 at 6:07 PM, Edward Ing <edward.ing@...> wrote:

saveFile n =
   do          cont <- (liftM fromJust) $ getInputFPS "file"
        let f = uploadDir ++ "/" ++ basename n
        liftIO $ BS.writeFile f cont
        return $ paragraph << ("Saved as " +++ anchor ! [href f] << f +++ ".")

Vs.

 

saveFile n =
do   cont <- getInputFPS "file"
       let f = uploadDir ++ "/" ++ basename n
      liftIO $ BS.writeFile f (fromJust cont)
      return $ paragraph << ("Saved as " +++ anchor ! [href f] << f +++ ".")

Consider the line x <- y in a do expression.  If y has type M a for some monad M, then x has type a.

So, let's say you have a value f :: Maybe Int, and you want to return the Int's stringification if it exists.  We can write this in these two ways:

do x <- f

   return (show x)

do x <- liftM show f

   return x

liftM :: (a -> b) -> (M a -> M b)   for any monad M.  That means if you want to apply a function to a value which is currently wrapped in a monad constructor, you need to "lift" it in.  liftM takes a function on ordinary values to a function on wrapped values.  But after you bind, you don't need to lift anymore.

Which of the two above styles to choose is a matter of style, and, in my code at least, varies from situation to situation.

That said, you can write both of these snippets as "fmap show f" or "show <$> f"  (where (<$>) is from Control.Applicative), which is how it would be done in practice.

Does that make sense?

Luke

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[Brandon S. Allbery KF8NH]

On Jun 27, 2009, at 20:07 , Edward Ing wrote:

> saveFile n =
>    do          cont <- (liftM fromJust) $ getInputFPS "file"
>         let f = uploadDir ++ "/" ++ basename n
>         liftIO $ BS.writeFile f cont
>         return $ paragraph << ("Saved as " +++ anchor ! [href f] <<  
> f +++ ".")
>
> saveFile n =
> do   cont <- getInputFPS "file"
>        let f = uploadDir ++ "/" ++ basename n
>       liftIO $ BS.writeFile f (fromJust cont)
>       return $ paragraph << ("Saved as " +++ anchor ! [href f] << f +
> ++ ".")
>
> 1) Why did the author choose to insert "liftM" in function saveFile?

It's because of where fromJust is being called.  In yours, it's being  
used at a place that expects a normal value, so you can just go ahead  
and use it.

The original is applying the fromJust inside of a monadic computation,  
as indicated by the (<-), so it needs to be lifted.  Some Haskell  
programmers use fmap (because most Monads are also Functors), others  
use liftM.  Both have the same effect:  given a monadic computation "m  
a", "liftM f" turns "f" into a function that operates on the enclosed  
"a" instead of the entire "m a".

--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@...
system administrator [openafs,heimdal,too many hats] allbery@...
electrical and computer engineering, carnegie mellon university    KF8NH




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[wren ng thornton]

Brandon S. Allbery KF8NH wrote:
> Some Haskell
> programmers use fmap (because most Monads are also Functors), others use
> liftM.  Both have the same effect:  given a monadic computation "m a",
> "liftM f" turns "f" into a function that operates on the enclosed "a"
> instead of the entire "m a".

That is, given the theory behind it all, every monad is a functor (note
the lower case); from which it follows that liftM == fmap. For
historical reasons the Monad typeclass does not require a Functor
instance, however, and so it's not the case that every Monad is also a
Functor (note the upper case).

The function liftM can be defined generically given definitions for
return and (>>=), so some prefer to use liftM to avoid the extra Functor
dependency. The function fmap can be given specialized definitions due
to overloading, so others prefer to use it for efficiency reasons. The
(<$>) function is just a symbolic name for fmap.


You'll also see the Applicative typeclass for "applicative functors".
Applicative does require a Functor instance, which is good. (And
actually, every monad is an applicative functor; though the Monad class
doesn't require Applicative either.) The function liftA can be defined
generically given definitions for pure and (<*>), and liftA == fmap as
well. The only reason anyone should use liftA is for defining a Functor
instance when they're too lazy to give a specialized implementation.


--
Live well,
~wren
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[Brandon S. Allbery KF8NH]

On Jul 2, 2009, at 17:59 , wren ng thornton wrote:
> Brandon S. Allbery KF8NH wrote:
>> Some Haskell programmers use fmap (because most Monads are also  
>> Functors), others use liftM.  Both have the same effect:  given a  
>> monadic computation "m a", "liftM f" turns "f" into a function that  
>> operates on the enclosed "a" instead of the entire "m a".
>
> That is, given the theory behind it all, every monad is a functor  
> (note the lower case); from

Yeh, I decided to bypass the whole "all monads are functors, but for  
Hysterical Raisins not all Monads are Functors" morass.

--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@...
system administrator [openafs,heimdal,too many hats] allbery@...
electrical and computer engineering, carnegie mellon university    KF8NH




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