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Today's Topics:
1. Clarifying $ vs parentheses (Josh Friedlander)
2. Re: Clarifying $ vs parentheses (Bob Ippolito)
3. Re: Clarifying $ vs parentheses (Francesco Ariis)
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Message: 1
Date: Thu, 20 Aug 2020 22:02:21 +0300
From: Josh Friedlander <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Clarifying $ vs parentheses
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I understand that in general $ is a) right-associative and b)
lowest-priority. But if so shouldn't these two be roughly the same?
λ take (succ 10) $ cycle "hello world"
"hello world"
But not this?
λ take $ succ 10 $ cycle "hello world"
<interactive>:20:8: error:
• No instance for (Enum ([Char] -> Int))
arising from a use of ‘succ’
(maybe you haven't applied a function to enough arguments?)
• In the expression: succ 10
In the second argument of ‘($)’, namely
‘succ 10 $ cycle "hello world"’
In the expression: take $ succ 10 $ cycle "hello world"
<interactive>:20:13: error:
• No instance for (Num ([Char] -> Int))
arising from the literal ‘10’
(maybe you haven't applied a function to enough arguments?)
• In the first argument of ‘succ’, namely ‘10’
In the expression: succ 10
In the second argument of ‘($)’, namely
‘succ 10 $ cycle "hello world"’
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Message: 2
Date: Thu, 20 Aug 2020 12:36:24 -0700
From: Bob Ippolito <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Clarifying $ vs parentheses
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Content-Type: text/plain; charset="utf-8"
Because the second one is:
take (succ 10 (cycle “hello world”))
On Thu, Aug 20, 2020 at 12:03 Josh Friedlander <[email protected]>
wrote:
> I understand that in general $ is a) right-associative and b)
> lowest-priority. But if so shouldn't these two be roughly the same?
>
> λ take (succ 10) $ cycle "hello world"
> "hello world"
>
> But not this?
> λ take $ succ 10 $ cycle "hello world"
>
> <interactive>:20:8: error:
> • No instance for (Enum ([Char] -> Int))
> arising from a use of ‘succ’
> (maybe you haven't applied a function to enough arguments?)
> • In the expression: succ 10
> In the second argument of ‘($)’, namely
> ‘succ 10 $ cycle "hello world"’
> In the expression: take $ succ 10 $ cycle "hello world"
>
> <interactive>:20:13: error:
> • No instance for (Num ([Char] -> Int))
> arising from the literal ‘10’
> (maybe you haven't applied a function to enough arguments?)
> • In the first argument of ‘succ’, namely ‘10’
> In the expression: succ 10
> In the second argument of ‘($)’, namely
> ‘succ 10 $ cycle "hello world"’
>
>
> _______________________________________________
>
> Beginners mailing list
>
> [email protected]
>
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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Message: 3
Date: Thu, 20 Aug 2020 21:40:31 +0200
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Clarifying $ vs parentheses
Message-ID: <20200820194031.GA30551@extensa>
Content-Type: text/plain; charset=utf-8
Hello Josh,
il 20 agosto 2020 alle 22:02 josh friedlander ha scritto:
> i understand that in general $ is a) right-associative and b)
> lowest-priority. but if so shouldn't these two be roughly the same?
>
> λ take (succ 10) $ cycle "hello world"
> "hello world"
>
> But not this?
> λ take $ succ 10 $ cycle "hello world"
>
> […]
λ> :info ($)
($) :: (a -> b) -> a -> b -- Defined in ‘GHC.Base’
infixr 0 $
So, since `$` is right associative, the expression
take $ succ 10 $ cycle "hello world"
becomes
take (succ 10 (cycle "hello world"))
`cycle "hello world"` makes sense, `succ 10` makes sense,
`succ 10 anotherArgument` does not.
Even `take someStuff` is probably not what you want, since take is usually
invoked with two arguments.
A useful intuition when you see ($) is `it will evaluate everything on
the right of it first`. This way, `not $ xx yy zz` looks right,
`take $ xx aa yy qq` less so.
Does this help?
—F
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