[Haskell-beginners] ghci and randomRs
Jeff C. Britton
jcb at iteris.com
Tue Sep 16 02:01:30 UTC 2014
I am trying to use randomRs at the ghci prompt like so
Prelude System.Random> take 10 $ (randomRs (1, 6) newStdGen)
but I get the following error
<interactive>:16:12:
Could not deduce (RandomGen (IO StdGen))
arising from a use of `randomRs'
from the context (Random a, Num a)
bound by the inferred type of it :: (Random a, Num a) => [a]
at <interactive>:16:1-37
In the second argument of `($)', namely
`(randomRs (1, 6) newStdGen)'
In the expression: take 10 $ (randomRs (1, 6) newStdGen)
In an equation for `it': it = take 10 $ (randomRs (1, 6) newStdGen)
I have tried a variety of options, like wrapping it in a "do" or adding type annotations.
Nothing seems to work.
-----Original Message-----
From: Beginners [mailto:beginners-bounces at haskell.org] On Behalf Of martin
Sent: Friday, September 12, 2014 10:05 AM
To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell
Subject: Re: [Haskell-beginners] How to add a "method" to a record
Am 09/10/2014 08:50 PM, schrieb Corentin Dupont:
> If the field "label" can be deduced from "payload", I recommend not to
> include it in your structure, because that would be redundant.
>
> Here how you could write it:
>
> data Foo pl = Foo { payload :: pl}
>
> labelInt :: Foo Int -> String
> labelInt (Foo a) = "Int payload:" ++ (show a)
>
> labelString :: Foo String -> String
> labelString (Foo a) = "String payload" ++ a
>
> You are obliged to define two separate label function, because "Foo Int" and "Foo String" are two completly separate types.
This is exactly my problem: Someone will use this type an define the type of pl. How can I know what type she'll use?
What I'd like to express is that whoever creates a concrete type should also provide the proper label function.
>
> On Wed, Sep 10, 2014 at 2:06 PM, martin <martin.drautzburg at web.de> wrote:
>
> Hello all
>
> if I have a record like
>
> data Foo pl = Foo {
> label :: String,
> payload :: pl
> }
>
> how can I create a similar type where I can populate label so it is not a plain string, but a function which
> operates on
> payload? Something like
>
> label (Foo pl) = show pl
>
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