Re: [Haskell-cafe] tips on using monads
Michael P Mossey wrote:
I've got one of those algorithms which threatens to march off the
right edge (in the words of Goerzen et al). I need something like a
State or Maybe monad, but this is inside the IO monad. So I presume I
need StateT or MaybeT. However, I'm still (slowly) learning about
monads from first principles. I thought I might present my code and
get some pointers... maybe someone could actually show me how to
rewrite it, which would be a neat way to see MaybeT and StateT in
action. I'm hoping to get anything from a one-line response to a
rewrite of my code. Anything will help.
Here's a version using ErrorT from mtl. I added some missing IO bits on
your types; the type error that remains is for you to fix :-) With
ErrorT you can use throwError when you want to break out of the block
and give back an error, which seems to fit what you were doing. The
downside is you have to add all these liftIO bits wherever you do a
plain IO computation.
insertNote :: NoteRecord - Connection - IO ()
insertNote nr conn = either putStrLn return = runErrorT
(do -- Check if it exists in the database already.
status - liftIO $ checkPreExistingText nr conn
when status $ throwError Skipping... text exists already.
-- Find best fit for all topics and source.
-- See type signatures below.
bestFitTopics - liftIO $ fitTopics nr conn
bestFitSource - liftIO $ fitSource nr conn
case any isNothing bestFitTopics of
True - throwError Error... some topic couldn't be matched.
False -
case bestFitSource of
Nothing - throwError Error.. source couldn't be matched.
_ - do b - liftIO $ isUserOkay nr bestFitTopics bestFitSource
when (not b) $ throwError Abort due to user request.
-- Create a new NoteRecord with matched
-- and validated topics/source.
let nrValidated =
nr { recordTopics = bestFitTopics
, recordSource = bestFitSource }
liftIO $ insertRow nrValidated conn
)
checkPreExistingText :: NoteRecord - Connection - IO Bool
fitTopics :: NoteRecord - Connection - IO [Maybe String]
fitSource :: NoteRecord - Connection - IO (Maybe String)
isUserOkay :: NoteRecord - [Maybe String] - Maybe String - IO Bool
insertRow :: NoteRecord - Connection - IO ()
Thanks,
Neil.
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Re: [Haskell-cafe] tips on using monads
I've got one of those algorithms which threatens to march off the right edge (in the words of Goerzen et al). I need something like a State or Maybe monad, but this is inside the IO monad. So I presume I need StateT or MaybeT. However, I'm still (slowly) learning about monads from first principles. I thought I might present my code and get some pointers... maybe someone could actually show me how to rewrite it, which would be a neat way to see MaybeT and StateT in action. I'm hoping to get anything from a one-line response to a rewrite of my code. Anything will help. Perhaps this is useful: http://www.haskell.org/haskellwiki/Equational_reasoning_examples#Coding_style:_indentation_creep_with_nested_Maybe Claus ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] tips on using monads
On Mon, May 18, 2009 at 3:08 AM, Neil Brown nc...@kent.ac.uk wrote:
With ErrorT you can use throwError when you want to break out of the
block and give back an error, which seems to fit what you were doing.
Of course, now that you are using throwError, you can remove a lot of
the extra indentation:
insertNote :: NoteRecord - Connection - IO ()
insertNote nr conn = either putStrLn return = runErrorT
(do -- Check if it exists in the database already.
status - liftIO $ checkPreExistingText nr conn
when status $ throwError Skipping... text exists already.
-- Find best fit for all topics and source.
-- See type signatures below.
bestFitTopics - liftIO $ fitTopics nr conn
bestFitSource - liftIO $ fitSource nr conn
when (any isNothing bestFitTopics) $ throwError Error... some topic
couldn't be matched.
when (isNothing bestFitSource) $ throwError Error.. source couldn't be
matched.
b - liftIO $ isUserOkay nr bestFitTopics bestFitSource
when (not b) $ throwError Abort due to user request.
-- Create a new NoteRecord with matched
-- and validated topics/source.
let nrValidated = nr { recordTopics = bestFitTopics, recordSource =
bestFitSource }
liftIO $ insertRow nrValidated conn
)
-- ryan
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Re: [Haskell-cafe] tips on using monads
I've got one of those algorithms which threatens to march off the right edge (in the words of Goerzen et al). I need something like a State or Maybe monad, but this is inside the IO monad. So I presume I need StateT or MaybeT. However, I'm still (sdlowly) learning about monads from first principles. I thought I might present my code and get some pointers... maybe someone could actually show me how to rewrite it, which would be a neat way to see MaybeT and StateT in action. I'm hoping to get anything from a one-line response to a rewrite of my code. Anything will help. Perhaps this is useful: http://www.haskell.org/haskellwiki/Equational_reasoning_examples#Coding_style:_indentation_creep_with_nested_Maybe I can't quite tell--is that example in the IO monad? Part of my difficulty is that I'm inside IO. I know how to do this with Maybe, except that I have to combine Maybe and IO (use MaybeT?) It was in the GHC.Conc.STM monad, so yes, it used a MaybeT and Control.Monad.Trans.MonadTrans's lift (btw, the MonadTrans docs only point to [1], but [2] might also be of interest, if rather more compact/terse). Claus [1] http://web.cecs.pdx.edu/~mpj/pubs/springschool.html [2] http://web.cecs.pdx.edu/~mpj/pubs/modinterp.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] tips on using monads
I've got one of those algorithms which threatens to march off the right edge
(in the words of Goerzen et al). I need something like a State or Maybe monad,
but this is inside the IO monad. So I presume I need StateT or MaybeT. However,
I'm still (slowly) learning about monads from first principles. I thought I
might present my code and get some pointers... maybe someone could actually show
me how to rewrite it, which would be a neat way to see MaybeT and StateT in
action. I'm hoping to get anything from a one-line response to a rewrite of my
code. Anything will help.
Here's the program:
{-
This is a program which starts with a document containing notes
about software requirements (in a particular format) and puts them
into a database. Notes include details such as the source of the
requirement (who gave it), the topic(s) to which it pertains, the
date, etc.
I have written a parser to take a text document typed up by me during a
meeting and parse it into a NoteRecord structure. Here is the
structure:
-}
data NoteRecord = NoteRecord {
recordSource :: String, -- Name of person who gave req.
recordDate :: [Int], -- Date in [year,month,date]
recordSourceType :: String, -- meeting, phone, email, etc.
recordBugNum :: Maybe Int,-- Bugzilla # (if relevant)
recordTopics :: [String], -- list of official topics pertaining
recordText :: String }-- the text of the note itself
deriving (Show)
{-
One other wrinkle. The source (person name) and topic must be one
of a set of pre-determined strings. A person has an official full name
which is stored in the database. Topics also have official descriptive
strings. If I wasn't clever, then the note, as I type it up,
must have the exact name and topic. But I hate trying to remember things
like that. So I have implemented a fuzzy string match system so
that I can type part of someone's name (or even misspell it) or part of
a topic string, and the system will find the best match to an official
string.
In pseudocode, the function to insert a note in the database must do this:
This function starts with a NoteRecord.
- If text already exists in the database, give an error and skip to end.
- Fuzzy-match strings to topics and source.
- If no potential match can be found to some of topics or source,
give error and skip to end.
- Ask user to confirm if the matched topics and source look okay.
- if user says no, skip to end.
- Actually insert the record.
-}
insertNote :: NoteRecord - Connection - IO ()
insertNote nr conn =
do -- Check if it exists in the database already.
status - checkPreExistingText nr conn
if status
then putStrLn Skipping... text exists already.
else
do -- Find best fit for all topics and source.
-- See type signatures below.
bestFitTopics - fitTopics nr conn
bestFitSource - fitSource nr conn
case any isNothing bestFitTopics of
True -
putStrLn Error... some topic couldn't be matched.
False -
case bestFitSource of
Nothing -
putStrLn Error.. source couldn't be matched.
_ -
do b - isUserOkay nr bestFitTopics bestFitSource
if b
then do
-- Create a new NoteRecord with matched
-- and validated topics/source.
nrValidated =
nr { recordTopics = bestFitTopics
, recordSource = bestFitSource }
insertRow nrValidated conn
else putStrLn Abort due to user request.
checkPreExistingText :: NoteRecord - Connection - Bool
-- There are multiple topics in the NoteRecord. For each one,
-- find the best fuzzy match, or indicate if there is no plausible
-- match at all.
fitTopics :: NoteRecord - Connection - [Maybe String]
-- There is one source. Try to find fuzzy match.
fitSource :: NoteRecord - Connection - Maybe String
-- Present user with all fuzzy matches and get a yes/no response if it's
-- okay to proceed.
isUserOkay :: NoteRecord - [Maybe String] - Maybe String - Bool
-- Do actual insert into database.
insertRow :: NoteRecord - Connection - IO ()
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