simple random numbers in Haskell

Random numbers are the kind of thing I use rarely enough that by the time I want to use them, I have forgotten the relevant details, but frequently enough that I get annoyed whenever it happens.

Hopefully these notes will be useful to somebody in a similar situation.

two things to know

(1) import System.Random

(2) randomIO :: Random a => IO a

The one function you really need to know about is randomIO (The type of this function is Random a => IO a. Don't worry if you do not understand the type; it suffices to know that it involves IO). In this example, we use and generate a random Int:

import System.Random

main =
 do r <- randomIO
    print (r + 1 :: Int)
    -- Note re the ':: Int' above: Haskell can't figure out from
    -- the context exactly what type of number you want, so we
    -- constrain it to Int

One neat feature is that you can randomly generate anything that implements the Random typeclass. In the example below, we generate a random Bool. Notice how we do not do anything differently, except to treat the result as a bool (i.e. by applying not to it)

import System.Random

main =
 do r <- randomIO
    print (not r)

A useful exercise, if you know about typeclasses, is to implement Random for one of your own types. The toEnum function may be useful.

more advanced stuff

1. you can use randomRIO :: Random a => (a,a) -> IO a to generate random numbers constrained within a range
2. Instead of using the functions randomIO and randomRIO, you can separate obtaining a random number generator, from using the generator. Doing so allows you to minimise your reliance on the IO monad. It also makes your code easier to debug, because you can opt to always pass the same generator to it and make life much more predictable. See the functions random and randomR for details.
3. a potentially handy trick is to generate an infinite list of random numbers, which you can then pass to a function. See the randoms function for details.

Edit: fixed s/randomR/randomIO/

Random numbers are the kind of thing I use rarely enough that by the time I want to use them, I have forgotten the relevant details, but frequently enough that I get annoyed whenever it happens.

Hopefully these notes will be useful to somebody in a similar situation.

two things to know

(1) import System.Random

(2) randomIO :: Random a => IO a

The one function you really need to know about is randomIO (The type of this function is Random a => IO a. Don't worry if you do not understand the type; it suffices to know that it involves IO). In this example, we use and generate a random Int:

import System.Random

main =
 do r <- randomIO
    print (r + 1 :: Int)
    -- Note re the ':: Int' above: Haskell can't figure out from
    -- the context exactly what type of number you want, so we
    -- constrain it to Int

One neat feature is that you can randomly generate anything that implements the Random typeclass. In the example below, we generate a random Bool. Notice how we do not do anything differently, except to treat the result as a bool (i.e. by applying not to it)

import System.Random

main =
 do r <- randomIO
    print (not r)

A useful exercise, if you know about typeclasses, is to implement Random for one of your own types. The toEnum function may be useful.

more advanced stuff

1. you can use randomRIO :: Random a => (a,a) -> IO a to generate random numbers constrained within a range
2. Instead of using the functions randomIO and randomRIO, you can separate obtaining a random number generator, from using the generator. Doing so allows you to minimise your reliance on the IO monad. It also makes your code easier to debug, because you can opt to always pass the same generator to it and make life much more predictable. See the functions random and randomR for details.
3. a potentially handy trick is to generate an infinite list of random numbers, which you can then pass to a function. See the randoms function for details.

Edit: fixed s/randomR/randomIO/

simple random numbers in Haskell

simply reading and writing UTF-8 in Haskell

A year and a half ago, I posted what seemed to be the simplest recipe for reading and writing UTF-8 in Haskell. In this post, I will provide an even simpler recipe, made possible by Eric Mertens' utf8-string package.

For those who are not familiar with Haskell, its internal representation for characters is Unicode, but for IO it effectively assumes that that it is reading and writing in the ISO8859-1 format. This used to be annoying for those of us who wanted to work with the UTF-8 encoding, but now there is a very simple solution, perfect for those of us who don't want to think too much and just get the job done.

the example

The sample problem from my last post was to take a UTF-8 encoded file as input, reverse all its lines, writing the results in the same file, with a ".rev" extension appended to its name. The solution might be self-explanatory if you are used to Haskell, but I will make some minor comments below, just in case.

import System.IO.UTF8
import Prelude hiding (readFile, writeFile)
import System.Environment (getArgs)

main =
 do args <- getArgs
    mapM_ reverseUTF8File args 

reverseUTF8File f =  
 do c <- readFile f
    writeFile (f ++ ".rev") $ reverseLines c

reverseLines = unlines . map reverse . lines

In the above code, we use some drop-in replacements for some System.IO functions. Some of these functions are also provided in the Prelude, so we must hide them so that they do not overlap with what we import. (Alternatively, we could import the UTF-8 ones qualified, which could be handy in contexts where we want the option of reading and writing in UTF-8 without committing to it). The rest is straightforward. Notice that we do not jump through any hoops whatsoever. In fact, you can pretty much take any pre-existing Haskell program that you have written and turn it into a UTF-8 version by changing the import statements.

Here are the results of running this script on a UTF-8 sampler:

)udrU( یتوہ ںیہن فیلکت ےھجم روا ںوہ اتکس اھک چناک ںیم 
)othsaP( يوږوخ هن ام هغه ،مش ېلړوخ هشيش هز 
)naeroK(요아않 지프아 도래그 .요어있 수 을먹 를리유 는나
)keerG( .ατοπίτ ωθάπ αν ςίρωχ άιλαυγ ανέμσαπσ ωάφ αν ώροπΜ
)cidnalecI / aksnelsÍ( .gim aðiem ða sseþ ná relg ðite teg  gÉ
)hsiloP( .izdokzs ein im i ,ołkzs ćśej ęgoM
)nainamoR( .etșenăr ăm un ae iș ălcits cnânăm ăs toP
)nainiarkU( .ьтидокшоп ен інем онов й ,олкш итсї ужом Я
)nainemrA( ։րենըչ տսիգնահնա իծնի և լետւո իկապա մանրԿ
)naigroeG( .ავიკტმ არა ად მაჭვ სანიმ
)idniH( .तह हन डप ईक स सउ झम ,ह तकस ख चक म
)werbeH( .יל קיזמ אל הזו תיכוכז לוכאל לוכי ינא
)hsiddiY( .ײװ טשינ רימ טוט סע ןוא זאלג ןסע ןעק ךיא
)cibarA( .ينملؤي ل اذه و جاجزلا لكأ ىلع رداق انأ
)esenapaJ( 。んせまけつ傷を私はれそ。すまれらべ食をスラ
)iahT( บจเนฉหใำทมไนมตแ ดไกจะรกนกนฉ
)slobmys ycnerruc( ₯·₮·₭·₫·₪·₩·₨·₧·₦·₥·₤·₣·₢·₡·¢·$·€·£·¥

The utf8-string package is available on HackageDB. Thanks to Eric M. for providing this little wrapper! It's a perfect example of the kind of thing which seems obvious... after somebody else has thought to do it.

A year and a half ago, I posted what seemed to be the simplest recipe for reading and writing UTF-8 in Haskell. In this post, I will provide an even simpler recipe, made possible by Eric Mertens' utf8-string package.

For those who are not familiar with Haskell, its internal representation for characters is Unicode, but for IO it effectively assumes that that it is reading and writing in the ISO8859-1 format. This used to be annoying for those of us who wanted to work with the UTF-8 encoding, but now there is a very simple solution, perfect for those of us who don't want to think too much and just get the job done.

the example

The sample problem from my last post was to take a UTF-8 encoded file as input, reverse all its lines, writing the results in the same file, with a ".rev" extension appended to its name. The solution might be self-explanatory if you are used to Haskell, but I will make some minor comments below, just in case.

import System.IO.UTF8
import Prelude hiding (readFile, writeFile)
import System.Environment (getArgs)

main =
 do args <- getArgs
    mapM_ reverseUTF8File args 

reverseUTF8File f =  
 do c <- readFile f
    writeFile (f ++ ".rev") $ reverseLines c

reverseLines = unlines . map reverse . lines

In the above code, we use some drop-in replacements for some System.IO functions. Some of these functions are also provided in the Prelude, so we must hide them so that they do not overlap with what we import. (Alternatively, we could import the UTF-8 ones qualified, which could be handy in contexts where we want the option of reading and writing in UTF-8 without committing to it). The rest is straightforward. Notice that we do not jump through any hoops whatsoever. In fact, you can pretty much take any pre-existing Haskell program that you have written and turn it into a UTF-8 version by changing the import statements.

Here are the results of running this script on a UTF-8 sampler:

)udrU( یتوہ ںیہن فیلکت ےھجم روا ںوہ اتکس اھک چناک ںیم 
)othsaP( يوږوخ هن ام هغه ،مش ېلړوخ هشيش هز 
)naeroK(요아않 지프아 도래그 .요어있 수 을먹 를리유 는나
)keerG( .ατοπίτ ωθάπ αν ςίρωχ άιλαυγ ανέμσαπσ ωάφ αν ώροπΜ
)cidnalecI / aksnelsÍ( .gim aðiem ða sseþ ná relg ðite teg  gÉ
)hsiloP( .izdokzs ein im i ,ołkzs ćśej ęgoM
)nainamoR( .etșenăr ăm un ae iș ălcits cnânăm ăs toP
)nainiarkU( .ьтидокшоп ен інем онов й ,олкш итсї ужом Я
)nainemrA( ։րենըչ տսիգնահնա իծնի և լետւո իկապա մանրԿ
)naigroeG( .ავიკტმ არა ად მაჭვ სანიმ
)idniH( .तह हन डप ईक स सउ झम ,ह तकस ख चक म
)werbeH( .יל קיזמ אל הזו תיכוכז לוכאל לוכי ינא
)hsiddiY( .ײװ טשינ רימ טוט סע ןוא זאלג ןסע ןעק ךיא
)cibarA( .ينملؤي ل اذه و جاجزلا لكأ ىلع رداق انأ
)esenapaJ( 。んせまけつ傷を私はれそ。すまれらべ食をスラ
)iahT( บจเนฉหใำทมไนมตแ ดไกจะรกนกนฉ
)slobmys ycnerruc( ₯·₮·₭·₫·₪·₩·₨·₧·₦·₥·₤·₣·₢·₡·¢·$·€·£·¥

The utf8-string package is available on HackageDB. Thanks to Eric M. for providing this little wrapper! It's a perfect example of the kind of thing which seems obvious... after somebody else has thought to do it.

simply reading and writing UTF-8 in Haskell