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Today's Topics:
1. Re: applicative default structure (David McBride)
----------------------------------------------------------------------
Message: 1
Date: Thu, 22 Dec 2016 10:46:45 -0500
From: David McBride <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] applicative default structure
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<CAN+Tr43yCmX9=ih-vehxwptfpw_5h4xs4bxgvsxh6xw7euc...@mail.gmail.com>
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I get a Num ambiguous constraint error as well, but it is merely not giving
you the rest of the errors.
Prelude> let a1 = pure (1::Int)
Prelude> let a2 = pure (1::Int)
Prelude> a1 == a2
• Ambiguous type variable ‘f0’ arising from a use of ‘a1’
prevents the constraint ‘(Applicative f0)’ from being solved.
Furthermore you can't compare an IO a with an IO a, so it will still cause
an error. But if you go:
Prelude>let a1 = 1
Prelude>let a2 = 1
Prelude>a1 == a2
True
Then it is totally fine with that.
On Thu, Dec 22, 2016 at 10:38 AM, Imants Cekusins <[email protected]> wrote:
> > what is the "default" structure if you don't specify any
>
> similar to:
>
> display::Show a => a -> String
> display = show
>
> fa::(Applicative f, Num a) => f a
> fa = pure 1
>
> f and a are *bounded* by Applicative and Num, so to say. No default. Or,
> it is typed however type is a bit broader than *
>
>
> > In this case it defaults f to IO and a to Int,
>
> does it though?
>
>
> Prelude> let a1 = pure 1
> Prelude> let a2 = pure 1
> Prelude> a1 == a2
>
> <interactive>:20:1: error:
> • Ambiguous type variable ‘a0’ arising from a use of ‘a1’
> prevents the constraint ‘(Num a0)’ from being solved.
>
>
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