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Today's Topics:
1. Re: Little question about "forall" (David McBride)
2. Re: Little question about "forall" (Baa)
3. Re: Stack could not find libHStransformers (Quentin Liu)
----------------------------------------------------------------------
Message: 1
Date: Thu, 7 Dec 2017 09:05:04 -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] Little question about "forall"
Message-ID:
<CAN+Tr4315xGNaqVg_kAK+XOhOC0CmnrHQSFetsYN=+6tadm...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
As to your original code, the only actual bug is that someA :: a is wrong.
The actual type of someA is someA :: C a => a, so you could have written
class C a where
someA :: a
f :: C a => a -> Int
f a =
let x :: C a => a
x = someA -- absolutely works
in 0
But remember that in x, 'a' is not the same as the 'a' in f, so you might
as well have written this
f :: C a => a -> Int
f a =
let x :: C b => b
x = someA -- also works
in 0
This is how it works. When you write
f :: C a => a -> ()
f =
let x = someA -- without ':: a'
()
The problem is that by not giving an explicit type to x, ghc infers a type
and for all intents and purposes it is as though you wrote
f :: C a => a -> ()
f =
let x :: C a1 => a1
x = someA
-- also if you had written x = some :: a, it just means you
effectively wrote
-- let x :: C a1 => a1
-- x = someA :: a
-- but 'a1' is not equivalent to 'a', so it errors.
()
And so it says hey wait, a and a1 could be different types. That is why
you often see ghc referring to 'a1' or 'a0' in error messages, because when
it infers a type it has to give a name to it in order to report on it.
What ScopedTypeVariables does is allow you to specify that for the entire
scope of this forall, 'a' in any type signature always refers to the same
type.
f :: forall a. C a => a -> ()
f = undefined
where
some :: a -- this is guaranteed to be the a from above.
some = undefined
somethingelse :: forall a. a -- this is a different a
somethingelse = undefined
some :: a -- this toplevel a is of course also different.
some = undefined
So you could just go
{-# LANGUAGE ScopedTypeVariables #-}
f :: forall a. C a => a -> Int
f a =
let
x :: a -- now the a is the same a as above, so the constraint C a is
understood.
x = someA
in 0
I really love the ScopedTypeVariables extension, and so do a lot of other
people. I honestly think it should be on by default because the pitfalls
of not having it on are a lot worse than the pitfalls of having it on. The
only pitfall I know of to having it on is that if you have a function with
tons of lets and / or wheres, you might not notice you had used the type
variable 'a' in two different incompatible places.
Hopefully all this makes sense.
On Thu, Dec 7, 2017 at 6:41 AM, Baa <aqua...@gmail.com> wrote:
> Hello everyone!
>
> Suppose I have:
>
> class C a where
> someA :: a
>
> f :: C a => a -> Int
> f a =
> let x = someA :: a in -- BUG!!
> 0
>
> BUG as I understand I due to `:: a` - this is another `a`, not the same as
> in `f` singature. But seems that it's the same `a` if `f` is the "method"
> of some instance. And such bug does not happen if I return it, so my
> question
> is how to create such `x` of type `a` in the body of the `f`? How to use
> `a` anywhere in the body of `f`? Is it possible?
>
> I found a way if I change signature to `forall a. C a => a -> Int`. But
> there
> are another questions in the case:
>
> - as I know all such params are under "forall" in Haskell by-default.
> Seems
> it's not true?
> - in method body seems they are under "forall" but in simple functions -
> not?
> - i'm not sure what exactly does this "forall", IMHO it unbounds early
> bound
> type's variables (parameters) by typechecker, but how `a` can be bound
> in the
> just begining of signature (where it occurs first time) ?!
>
> As for me, looks that w/o "forall" all `a`, `b`, `c`, etc in the body of
> simple functions always are different types. But w/ "forall" Haskell
> typechecker
> makes something like (on type-level):
>
> let a = ANY-TYPE
> in
> ... here `a` can occur and will be bound to ANY-TYPE
>
> Where am I right and where am I wrong? :)
>
> ===
> Best regards, Paul
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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Message: 2
Date: Thu, 7 Dec 2017 17:31:54 +0200
From: Baa <aqua...@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Little question about "forall"
Message-ID: <20171207173154.5cfc935c@Pavel>
Content-Type: text/plain; charset=UTF-8
Hello, David!
> But remember that in x, 'a' is not the same as the 'a' in f, so you
Exactly, and I often hit errors like "Can't match rigid type 'M a' with
rigid type 'M a1'"! So, the question comes down to "forall" and
"ScopedTypeVariables". Did I understand you correctly:
this extension expands type-variable scope to whole function body. But
what sort of type vairables? Only under "forall ...", right ?
And another question: type-variables under "forall" - they are rigid?
Or free? I can't understand the terminology. I thought early that they
are called "rigid", but in documentation of "ScopedTypeVariables"
extension they are called "free". IMHO symbol ∀ means "unbound", so
"free". But I often hit errors when GHC says something about rigid, so
I'm not sure - what is rigid? Bound?
And last: why I need "forall a." in beginning of signature while
Haskell assumes "forall" for all type variables by default? Or I'm not
right here...
> As to your original code, the only actual bug is that someA :: a is
> wrong. The actual type of someA is someA :: C a => a, so you could
> have written
>
> class C a where
> someA :: a
>
> f :: C a => a -> Int
> f a =
> let x :: C a => a
> x = someA -- absolutely works
> in 0
>
>
> But remember that in x, 'a' is not the same as the 'a' in f, so you
> might as well have written this
>
> f :: C a => a -> Int
> f a =
> let x :: C b => b
> x = someA -- also works
> in 0
>
> This is how it works. When you write
>
> f :: C a => a -> ()
> f =
> let x = someA -- without ':: a'
> ()
>
> The problem is that by not giving an explicit type to x, ghc infers a
> type and for all intents and purposes it is as though you wrote
>
> f :: C a => a -> ()
> f =
> let x :: C a1 => a1
> x = someA
> -- also if you had written x = some :: a, it just means you
> effectively wrote
> -- let x :: C a1 => a1
> -- x = someA :: a
> -- but 'a1' is not equivalent to 'a', so it errors.
> ()
>
> And so it says hey wait, a and a1 could be different types. That is
> why you often see ghc referring to 'a1' or 'a0' in error messages,
> because when it infers a type it has to give a name to it in order to
> report on it.
>
> What ScopedTypeVariables does is allow you to specify that for the
> entire scope of this forall, 'a' in any type signature always refers
> to the same type.
>
> f :: forall a. C a => a -> ()
> f = undefined
> where
> some :: a -- this is guaranteed to be the a from above.
> some = undefined
>
> somethingelse :: forall a. a -- this is a different a
> somethingelse = undefined
>
> some :: a -- this toplevel a is of course also different.
> some = undefined
>
> So you could just go
> {-# LANGUAGE ScopedTypeVariables #-}
>
> f :: forall a. C a => a -> Int
> f a =
> let
> x :: a -- now the a is the same a as above, so the constraint C
> a is understood.
> x = someA
> in 0
>
> I really love the ScopedTypeVariables extension, and so do a lot of
> other people. I honestly think it should be on by default because
> the pitfalls of not having it on are a lot worse than the pitfalls of
> having it on. The only pitfall I know of to having it on is that if
> you have a function with tons of lets and / or wheres, you might not
> notice you had used the type variable 'a' in two different
> incompatible places.
>
> Hopefully all this makes sense.
>
>
>
> On Thu, Dec 7, 2017 at 6:41 AM, Baa <aqua...@gmail.com> wrote:
>
> > Hello everyone!
> >
> > Suppose I have:
> >
> > class C a where
> > someA :: a
> >
> > f :: C a => a -> Int
> > f a =
> > let x = someA :: a in -- BUG!!
> > 0
> >
> > BUG as I understand I due to `:: a` - this is another `a`, not the
> > same as in `f` singature. But seems that it's the same `a` if `f`
> > is the "method" of some instance. And such bug does not happen if I
> > return it, so my question
> > is how to create such `x` of type `a` in the body of the `f`? How
> > to use `a` anywhere in the body of `f`? Is it possible?
> >
> > I found a way if I change signature to `forall a. C a => a -> Int`.
> > But there
> > are another questions in the case:
> >
> > - as I know all such params are under "forall" in Haskell
> > by-default. Seems
> > it's not true?
> > - in method body seems they are under "forall" but in simple
> > functions - not?
> > - i'm not sure what exactly does this "forall", IMHO it unbounds
> > early bound
> > type's variables (parameters) by typechecker, but how `a` can be
> > bound in the
> > just begining of signature (where it occurs first time) ?!
> >
> > As for me, looks that w/o "forall" all `a`, `b`, `c`, etc in the
> > body of simple functions always are different types. But w/
> > "forall" Haskell typechecker
> > makes something like (on type-level):
> >
> > let a = ANY-TYPE
> > in
> > ... here `a` can occur and will be bound to ANY-TYPE
> >
> > Where am I right and where am I wrong? :)
> >
> > ===
> > Best regards, Paul
> > _______________________________________________
> > Beginners mailing list
> > Beginners@haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >
------------------------------
Message: 3
Date: Thu, 7 Dec 2017 10:42:08 -0500
From: Quentin Liu <quentin.liu.0...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Stack could not find
libHStransformers
Message-ID: <02ba63c5-632b-4175-b496-086b6c20d8cc@Spark>
Content-Type: text/plain; charset="utf-8"
I am using ghc download and managed by stack. My stack is of the latest
version, which is 1.6.1.
Regards,
Qingbo Liu
On Dec 7, 2017, 01:56 -0500, Michael Snoyman <mich...@snoyman.com>, wrote:
> What version of Stack are you using (stack --version), and how did you
> install Stack and GHC?
>
> It looks like you're using a system-wide GHC installation, which (for reasons
> like this) we by default no longer use by default in recent Stack releases.
>
> Upgrading to the latest Stack (via `stack upgrade` or following the
> instructions at [1]) will probably solve the problem.
>
> [1] https://haskell-lang.org/get-started/osx
>
> > On Thu, Dec 7, 2017 at 8:43 AM, Quentin Liu <quentin.liu.0...@gmail.com>
> > wrote:
> > > Hi,
> > >
> > > I was trying to use Parsec and imported Text.ParserCombinators.Parsec. I
> > > could call “stack ghc” to compile the file, but when I loaded the file
> > > into ghci and called any function, ghci would report errors
> > > > can't load .so/.DLL for:
> > > > /Users/HereWegoR/.stack/snapshots/x86_64-osx/lts-8.8/8.0.2/lib/x86_64-osx-ghc-8.0.2/libHSmtl-2.2.1-BLKBelFsPB3BoFeSWSOYj6-ghc8.0.2.dylib
> > > >
> > > > (dlopen(/Users/HereWegoR/.stack/snapshots/x86_64-osx/lts-8.8/8.0.2/lib/x86_64-osx-ghc-8.0.2/libHSmtl-2.2.1-BLKBelFsPB3BoFeSWSOYj6-ghc8.0.2.dylib,
> > > > 5): Library not loaded:
> > > > /usr/local/opt/ghc/lib/ghc-8.0.2/transformers-0.5.2.0/libHStransformers-0.5.2.0-ghc8.0.2.dylib
> > > > Referenced from:
> > > > /Users/HereWegoR/.stack/snapshots/x86_64-osx/lts-8.8/8.0.2/lib/x86_64-osx-ghc-8.0.2/libHSmtl-2.2.1-BLKBelFsPB3BoFeSWSOYj6-ghc8.0.2.dylib
> > > > Reason: image not found)
> > > >
> > >
> > > I tried to install transformers by calling `stack install transformers`
> > > but the problem still persisted. Is it because the version of
> > > transformers in my repo is different from the one referenced by `mtl`
> > > package? How should I fix it?
> > >
> > > Regards,
> > > Qingbo Liu
> > >
> > > _______________________________________________
> > > Beginners mailing list
> > > Beginners@haskell.org
> > > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> > >
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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