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Today's Topics:
1. applicative default structure (Ovidiu Deac)
2. Re: applicative default structure (Imants Cekusins)
3. Re: applicative default structure (Ovidiu Deac)
4. Re: applicative default structure (David McBride)
5. Re: applicative default structure (Imants Cekusins)
6. Re: applicative default structure (Imants Cekusins)
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Message: 1
Date: Thu, 22 Dec 2016 17:06:48 +0200
From: Ovidiu Deac <ovidiud...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] applicative default structure
Message-ID:
<cakvse7umcsj8f0d+o7pye5jbc_chp9bapetqrotnyuvgcbh...@mail.gmail.com>
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This code is using Applicative Maybe. The structure is provided by Maybe
and the value 1 is wrapped in this structure. No surprises here.
Prelude> pure 1 :: Maybe Int
Just 1
Prelude> :t (pure 1 :: Maybe Int)
(pure 1 :: Maybe Int) :: Maybe Int
...but can somebody explain the type of x below?
Prelude> x = pure 1
Prelude> x
1
Prelude> :t x
x :: (Applicative f, Num a) => f a
What is f here?
Thanks!
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Message: 2
Date: Thu, 22 Dec 2016 16:13:37 +0100
From: Imants Cekusins <ima...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] applicative default structure
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> What is f here?
anything Applicative:
Prelude> let a1 = pure 1
Prelude> let a2 = pure 1
Prelude> (a1::Maybe Int) == a2
True
Prelude> (a1::Maybe Float) == a2
True
Prelude> (a1::Either String Float) == a2
True
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Message: 3
Date: Thu, 22 Dec 2016 17:18:16 +0200
From: Ovidiu Deac <ovidiud...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] applicative default structure
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My understanding is that x can take any form required for type-inference.
That's fine but what is the "default" structure if you don't specify any?
On Thu, Dec 22, 2016 at 5:13 PM, Imants Cekusins <ima...@gmail.com> wrote:
> > What is f here?
>
> anything Applicative:
>
> Prelude> let a1 = pure 1
> Prelude> let a2 = pure 1
>
> Prelude> (a1::Maybe Int) == a2
> True
> Prelude> (a1::Maybe Float) == a2
> True
> Prelude> (a1::Either String Float) == a2
> True
>
>
>
>
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> Beginners mailing list
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Message: 4
Date: Thu, 22 Dec 2016 10:33:41 -0500
From: David McBride <toa...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] applicative default structure
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<CAN+Tr430TySk4wo+W87d=naxqkfkexrkvq+n8aj+ssmqhps...@mail.gmail.com>
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Rather than bailing with an instance error for both Applicative and Num,
which is the technically the right way, and the way that it used to be in
the dark ages of ghci. Instead it chooses types which are probably what
you wanted. In this case it defaults f to IO and a to Int, and then runs
it.
You can read more about type defaulting in ghci here:
https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html
On Thu, Dec 22, 2016 at 10:18 AM, Ovidiu Deac <ovidiud...@gmail.com> wrote:
> My understanding is that x can take any form required for type-inference.
> That's fine but what is the "default" structure if you don't specify any?
>
> On Thu, Dec 22, 2016 at 5:13 PM, Imants Cekusins <ima...@gmail.com> wrote:
>
>> > What is f here?
>>
>> anything Applicative:
>>
>> Prelude> let a1 = pure 1
>> Prelude> let a2 = pure 1
>>
>> Prelude> (a1::Maybe Int) == a2
>> True
>> Prelude> (a1::Maybe Float) == a2
>> True
>> Prelude> (a1::Either String Float) == a2
>> True
>>
>>
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners@haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
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>
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Message: 5
Date: Thu, 22 Dec 2016 16:38:09 +0100
From: Imants Cekusins <ima...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] applicative default structure
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> 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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Message: 6
Date: Thu, 22 Dec 2016 16:45:58 +0100
From: Imants Cekusins <ima...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] applicative default structure
Message-ID:
<cap1qinyrs_pbxzqcuhqdizsjcvhe_-gb8aemiie5wznqxlv...@mail.gmail.com>
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> type is a bit broader than *
ignore this phrase. it is incorrect.
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