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Today's Topics:
1. Multiple parameters vs anonymous syntax (Wink Saville)
2. Re: Multiple parameters vs anonymous syntax (David McBride)
3. Re: Multiple parameters vs anonymous syntax (Wink Saville)
----------------------------------------------------------------------
Message: 1
Date: Tue, 17 Oct 2017 17:28:20 +0000
From: Wink Saville <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Multiple parameters vs anonymous syntax
Message-ID:
<CAKk8isqRNG09V3xpBFN=w+aSL=w_zwheofe6w12bq6jf9cl...@mail.gmail.com>
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I'm going through "Haskell Programming from first principles" and in
section 7.3 Anonymous Functions there is an exercise on converting multiple
parameters to anonymous functions, and it asks:
1. Which (two or more) of the following are equivalent?
mTh1 x y z = x * y * z
mTh2 x y = \z -> x * y * z
mTh3 x = \y -> \z -> x * y * z
mTh4 = \x -> \y -> \z -> x * y * z
So I created a file, anon.hs (attached):
module Anon where
mTh1 x y z = x * y * z
mTh2 x y = \z -> x * y * z
mTh3 x = \y -> \z -> x * y * z
mTh4 = \x -> \y -> \z -> x * y * z
I load that into ghci and check the function types:
$ ghci anon.hs
GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Anon ( anon.hs, interpreted )
Ok, 1 module loaded.
*Anon> :t mTh1
mTh1 :: Num a => a -> a -> a -> a
*Anon> :t mTh2
mTh2 :: Num a => a -> a -> a -> a
*Anon> :t mTh3
mTh3 :: Num a => a -> a -> a -> a
*Anon> :t mTh4
mTh4 :: Integer -> Integer -> Integer -> Integer
Why is mTh4 different from the rest?
On the flip side If I enter "mTh4 = \x -> \y -> \z -> x * y * z" directly
in ghci command line then it has same type as the others:
$ ghci
GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help
Prelude> mTh4 = \x -> \y -> \z -> x * y * z
Prelude> :t mTh4
mTh4 :: Num a => a -> a -> a -> a
-- Wink
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Message: 2
Date: Tue, 17 Oct 2017 13:52:48 -0400
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] Multiple parameters vs anonymous
syntax
Message-ID:
<can+tr40x5hcac0mvcg0-vyns_yeajn5wunyepwtzl0yxms3...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
It is because of NoMomomorphismRestriction
>let mTh4 = \x -> \y -> \z -> x * y * z
>:t mTh4
mTh4 :: Integer -> Integer -> Integer -> Integer
>:set -XNoMonomorphismRestriction
>let mTh4 = \x -> \y -> \z -> x * y * z
>:t mTh4
mTh4 :: Num a => a -> a -> a -> a
I'm not going into it too deeply, as it is somewhat involved and you can
read about it but I believe when a function "takes no arguments", it is
allowed to specialize polymorphic variables to defaults, and due to the Num
constraint it chooses Integer.
On Tue, Oct 17, 2017 at 1:28 PM, Wink Saville <[email protected]> wrote:
> I'm going through "Haskell Programming from first principles" and in
> section 7.3 Anonymous Functions there is an exercise on converting multiple
> parameters to anonymous functions, and it asks:
>
> 1. Which (two or more) of the following are equivalent?
>
> mTh1 x y z = x * y * z
> mTh2 x y = \z -> x * y * z
> mTh3 x = \y -> \z -> x * y * z
> mTh4 = \x -> \y -> \z -> x * y * z
>
> So I created a file, anon.hs (attached):
>
> module Anon where
>
> mTh1 x y z = x * y * z
> mTh2 x y = \z -> x * y * z
> mTh3 x = \y -> \z -> x * y * z
> mTh4 = \x -> \y -> \z -> x * y * z
>
> I load that into ghci and check the function types:
>
> $ ghci anon.hs
> GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help
> [1 of 1] Compiling Anon ( anon.hs, interpreted )
> Ok, 1 module loaded.
> *Anon> :t mTh1
> mTh1 :: Num a => a -> a -> a -> a
> *Anon> :t mTh2
> mTh2 :: Num a => a -> a -> a -> a
> *Anon> :t mTh3
> mTh3 :: Num a => a -> a -> a -> a
> *Anon> :t mTh4
> mTh4 :: Integer -> Integer -> Integer -> Integer
>
> Why is mTh4 different from the rest?
>
>
> On the flip side If I enter "mTh4 = \x -> \y -> \z -> x * y * z" directly
> in ghci command line then it has same type as the others:
>
> $ ghci
> GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help
> Prelude> mTh4 = \x -> \y -> \z -> x * y * z
> Prelude> :t mTh4
> mTh4 :: Num a => a -> a -> a -> a
>
>
> -- Wink
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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------------------------------
Message: 3
Date: Tue, 17 Oct 2017 18:30:11 +0000
From: Wink Saville <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Multiple parameters vs anonymous
syntax
Message-ID:
<cakk8isro-nxuzb-nozgnvcbksnmzq3sfqj6ykf7gycmx0ft...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Thank you, my head's spinning just starting to read [1] :)
BTW, you led me to find the answer to why ghci was different in the
interactive mode vs loading. I discovered there are two sets of options
used for ghci which are controlled by ":set" and ":seti", [2]. And in the
interactive mode we see -XNoMonomorphismRestriction is in effect:
Prelude> :seti
base language is: Haskell2010
with the following modifiers:
-XExtendedDefaultRules
-XNoMonomorphismRestriction
-XNondecreasingIndentation
GHCi-specific dynamic flag settings:
other dynamic, non-language, flag settings:
-fimplicit-import-qualified
warning settings:
But it's not in the "non-interactive" mode:
Prelude> :set
options currently set: none.
base language is: Haskell2010
with the following modifiers:
-XNondecreasingIndentation
GHCi-specific dynamic flag settings:
other dynamic, non-language, flag settings:
-fimplicit-import-qualified
warning settings:
[1]: https://wiki.haskell.org/Monomorphism_restriction
[2]:
https://downloads.haskell.org/~ghc/7.8.4/docs/html/users_guide/ghci-set.html
On Tue, Oct 17, 2017 at 10:53 AM David McBride <[email protected]> wrote:
> It is because of NoMomomorphismRestriction
>
> >let mTh4 = \x -> \y -> \z -> x * y * z
>
> >:t mTh4
> mTh4 :: Integer -> Integer -> Integer -> Integer
> >:set -XNoMonomorphismRestriction
> >let mTh4 = \x -> \y -> \z -> x * y * z
>
> >:t mTh4
> mTh4 :: Num a => a -> a -> a -> a
>
> I'm not going into it too deeply, as it is somewhat involved and you can
> read about it but I believe when a function "takes no arguments", it is
> allowed to specialize polymorphic variables to defaults, and due to the Num
> constraint it chooses Integer.
>
> On Tue, Oct 17, 2017 at 1:28 PM, Wink Saville <[email protected]> wrote:
>
>> I'm going through "Haskell Programming from first principles" and in
>> section 7.3 Anonymous Functions there is an exercise on converting multiple
>> parameters to anonymous functions, and it asks:
>>
>> 1. Which (two or more) of the following are equivalent?
>>
>> mTh1 x y z = x * y * z
>> mTh2 x y = \z -> x * y * z
>> mTh3 x = \y -> \z -> x * y * z
>> mTh4 = \x -> \y -> \z -> x * y * z
>>
>> So I created a file, anon.hs (attached):
>>
>> module Anon where
>>
>> mTh1 x y z = x * y * z
>> mTh2 x y = \z -> x * y * z
>> mTh3 x = \y -> \z -> x * y * z
>> mTh4 = \x -> \y -> \z -> x * y * z
>>
>> I load that into ghci and check the function types:
>>
>> $ ghci anon.hs
>> GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help
>> [1 of 1] Compiling Anon ( anon.hs, interpreted )
>> Ok, 1 module loaded.
>> *Anon> :t mTh1
>> mTh1 :: Num a => a -> a -> a -> a
>> *Anon> :t mTh2
>> mTh2 :: Num a => a -> a -> a -> a
>> *Anon> :t mTh3
>> mTh3 :: Num a => a -> a -> a -> a
>> *Anon> :t mTh4
>> mTh4 :: Integer -> Integer -> Integer -> Integer
>>
>> Why is mTh4 different from the rest?
>>
>>
>> On the flip side If I enter "mTh4 = \x -> \y -> \z -> x * y * z" directly
>> in ghci command line then it has same type as the others:
>>
>> $ ghci
>> GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help
>> Prelude> mTh4 = \x -> \y -> \z -> x * y * z
>> Prelude> :t mTh4
>> mTh4 :: Num a => a -> a -> a -> a
>>
>>
>> -- Wink
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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