[Haskell-beginners] Multiple parameters vs anonymous syntax

[Wink Saville]

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 <toad3k at gmail.com> 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 <wink at saville.com> 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
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20171017/255bce9e/attachment-0001.html>