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Today's Topics:
1. Re: Unit conversion (aditya siram)
2. Using Iteratees to write to a file (Richard Wallace)
----------------------------------------------------------------------
Message: 1
Date: Fri, 18 Mar 2011 23:11:22 -0500
From: aditya siram <aditya.si...@gmail.com>
Subject: Re: [Haskell-beginners] Unit conversion
To: Tom Murphy <amin...@gmail.com>
Cc: Magnus Therning <mag...@therning.org>, beginners@haskell.org
Message-ID:
<AANLkTim9n4iyy6DZMOt4oab_6C1nqVb6SEzYAf8Z=5...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
How about something like this?
import Data.Monoid
data Volumes = Teaspoon | Tablespoon | Slice | FluidOunces
type Quantity = (Int, Volumes)
instance Monoid Quantity where
mempty = (0, Teaspoon)
mappend a b = ((toTeaspoon a) + (toTeaspoon b), Teaspoon)
toTeaspoon :: Quantity -> Int
toTeaspoon (a, Teaspoon) = a
toTeaspoon (a, Tablespoon) = undefined
toTeaspoon (a, Slice) = undefined
toTeaspoon (a, FluidOunces) = undefined
-deech
On Fri, Mar 18, 2011 at 10:02 PM, Tom Murphy <amin...@gmail.com> wrote:
> On Fri, Mar 18, 2011 at 5:02 AM, Magnus
> Therning?<mag...@therning.org>?wrote:
>>
>> On Thu, Mar 17, 2011 at 21:17, Tom Murphy <amin...@gmail.com> wrote:
>> > Hi All,
>> > ?? ? Is there a good way to easily convert between units?
>> > ?? ? For example, let's say I have a data type:
>> > ?? ? data Volumes = Teaspoon | Tablespoon | Slice | FluidOunces
>> > ?? ? ... and I want to define an infix function '<+>', which adds
>> > together
>> > amounts of food:
>> > ?? ? (1, Slice, cake) <+> (1, Volume Tablespoon, sugar) <+> (32, Volume
>> > FluidOunces, almondMilk)
>> > ?? ? Which would return:
>> > ?? ? (3200, Teaspoons)
>> > ?? ? What is the most efficient way to define equivalency/conversion
>> > between
>> > these measures?
>> > ?? ? I remember an interesting method for celsius-farenheit conversion
>> > in
>> > Higher-Order Perl, using function composition, but that was between only
>> > 2
>> > units. I'd like something where I don't have to provide n^2 definitions.
>>
>> I wrote two blog posts on something that sounds related:
>> http://therning.org/magnus/archives/354
>> http://therning.org/magnus/archives/355
>>
>> Maybe they could help.
>>
>> /M
>>
>> --
>> Magnus Therning ? ? ? ? ? ? ? ? ? ? ?OpenPGP: 0xAB4DFBA4
>> email:?mag...@therning.org?? jabber:?mag...@therning.org
>> twitter: magthe ? ? ? ? ? ? ??http://therning.org/magnus
>
>
>
>
> So given
> data Measure = M Volume Float
> ??deriving (Show, Eq)
> data Volume = Slice
> ?? ? ? ? ? ?| FluidOunce
> ?? ? ? ? ? ?| Tablespoon
> ?? ? ? ? ? ?| Teaspoon
> ??deriving (Show, Eq)
> Method one and two would be to convert all units to the smallest unit:
> toTsp ? ? ? ? ? ? ? ? ? ? ? ? ? :: Measure -> Measure
> [...]
> toTsp (M Slice a) ? ? ? ? ?= M Teaspoon (a * 350)
> toTsp (M FluidOunce a) = M Teaspoon (a * 34)
> [...]
> Perform the addition, then convert it to a more appropriate measure (back to
> Cake Slices...).
> ?? ? It seems that there's a potential loss of precision, if I have to for
> example convert Megaliter measures to Teaspoons.
>
> Method three involves using typeclasses to define common "meeting points"
> for the units.
> So, we say that if we're adding Slices and FluidOunces, both measures should
> be converted to FluidOunces before the addition.
> The problem there is that, given n measurement units, we have to declare n^2
> typeclass instances by hand.
>
> Another way that I can see to do this is to define a "chain" of conversions.
> I'd then "convert down" only to the smallest-common unit.
> So I could define
> (warning: extreme kludge ahead)
> a series of "step-down"s and "step-up"s:
> Given a largest-to-smallest ordering of Slice, FluidOunce, Tablespoon,
> Teaspoon, I can define:
> stepDown ? ? ? ? ? ? ? ? ? ? ? ? ? :: Measure -> Measure
> stepDown (M Slice a) ? ? ? ? ?= M FluidOunce (a * 120)
> stepDown (M FluidOunce a) = M Tablespoon (a * 2)
> stepUp ? (M FluidOunce a) ? = M Slice (a / 120)
> stepUp ? (M Tablespoon a) ? = M FluidOunce (a / 2)
> [...]
>
> and then a function to "step" as far down as needed:
> convertDown :: Measure -> Volume -> Measure
> convertDown (M unit measure) goalUnit
> ?? ? ? ? ? ? ? ? ? ? | unit == goalUnit = M unit measure
> ?? ? ? ? ? ? ? ? ? ? | otherwise ? ? ? ?= fun5 (stepDown (M unit measure))
> ?goalUnit
>
>
> And then if I wanted to add a Slice of cake to a Tablespoon of almond milk,
> I'd have to find the smaller unit, convert down, perform the math and then
> do a "convert up" operation.
> An added benefit to this method is that defining a unit smaller than the
> current smallest wouldn't involve revising the "base unit" measures.
> This seems like a pretty common C.S. problem; I'm surprised I haven't been
> able to find more resources about it.
> Thanks for your time!
> Tom
> P.S. Thank you, Magnus, for those resources!
> _______________________________________________
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> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
>
------------------------------
Message: 2
Date: Sat, 19 Mar 2011 01:01:46 -0700
From: Richard Wallace <rwall...@thewallacepack.net>
Subject: [Haskell-beginners] Using Iteratees to write to a file
To: beginners@haskell.org
Message-ID:
<AANLkTi=hu5m1pwpmf-2xfff78eclpvnxdxnlso65w...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Hello everyone,
I've been reading "Iteratee: Teaching an Old Fold New Tricks" from the
Monad Reader Issue 16 and am having a problem figuring out how to use
the streamToFile function. Specifically, I can't figure out how to
feed the Iteratee data to write to the file. If I run
ghci> runIter $ streamToFile "/tmp/stream"
Then the file is created but not populated with any data - which is
natural since I didn't give it any data. I have no clue how to give
it data though. I'd really appreciate it if someone could show me how
to write "Hello world!" or something else equally mundane to get me
going. I've been banging my head on this for hours now.
Thanks,
Rich
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