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Today's Topics:
1. $ versus . (Lawrence Bottorff)
2. Re: $ versus . (Bob Ippolito)
3. Re: $ versus . (Kim-Ee Yeoh)
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Message: 1
Date: Mon, 25 Jan 2021 12:16:05 -0600
From: Lawrence Bottorff <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] $ versus .
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<cafahfsud_yhwd4w_4xjeskubkxt32bxxr5c6kfu+o2vcqhm...@mail.gmail.com>
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I've got this
> init $ tail [1,2,3]
[2]
and this
> chopEnds = init $ tail
> chopEnds [1,2,3]
[1,2]
What happened? Why is it not just init $ tail [1,2,3] ?
This works fine
> chopEnds2 = init . tail
> chopEnds2 [1,2,3]
[2]
What am I missing?
LB
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Message: 2
Date: Mon, 25 Jan 2021 11:18:20 -0800
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] $ versus .
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I think what you're missing is what you actually typed in the first case.
This is a type error, it will not compile or run:
chopEnds = init $ tail
The $ operator can always be rewritten as parentheses, in this case:
chopEnds = init (tail)
Which has the same incorrectly typed meaning as:
chopEnds = init tail
The "result" you pasted looks equivalent to:
chopEnds = init
Perhaps this is what you typed? In this case the argument tail it will
shadow the existing binding of the Prelude tail, which would be confusing
so with -Wall it would issue a compiler warning:
chopEnds tail = init $ tail
<interactive>:2:10: warning: [-Wname-shadowing]
This binding for ‘tail’ shadows the existing binding
imported from ‘Prelude’ (and originally defined in ‘GHC.List’)
On Mon, Jan 25, 2021 at 10:16 AM Lawrence Bottorff <[email protected]>
wrote:
> I've got this
>
> > init $ tail [1,2,3]
> [2]
>
> and this
>
> > chopEnds = init $ tail
> > chopEnds [1,2,3]
> [1,2]
>
> What happened? Why is it not just init $ tail [1,2,3] ?
>
> This works fine
>
> > chopEnds2 = init . tail
> > chopEnds2 [1,2,3]
> [2]
>
> What am I missing?
>
> LB
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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Message: 3
Date: Tue, 26 Jan 2021 08:58:11 +0700
From: Kim-Ee Yeoh <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] $ versus .
Message-ID:
<CAPY+ZdS6WqRLjrZMe4NhP7jAWQOWtsG-AZy3=_4xhz6f5l3...@mail.gmail.com>
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init $ tail [1,2,3]
= init (tail ([1,2,3])) -- a la Lisp
Now, functional programming is awesomest at abstractions. What if we could
abstract out "init (tail"?
Then we could write
chopEnds = init (tail
But that looks weird. It's only got the left half of a parens pair!
Does that explain why you should not expect the same result?
A separate question is why the compiler even type-checks "init $ tail" in
the first place. What do you think is going on there?
On Tue, Jan 26, 2021 at 1:16 AM Lawrence Bottorff <[email protected]> wrote:
> I've got this
>
> > init $ tail [1,2,3]
> [2]
>
> and this
>
> > chopEnds = init $ tail
> > chopEnds [1,2,3]
> [1,2]
>
> What happened? Why is it not just init $ tail [1,2,3] ?
>
> This works fine
>
> > chopEnds2 = init . tail
> > chopEnds2 [1,2,3]
> [2]
>
> What am I missing?
>
> LB
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
--
-- Kim-Ee
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