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Today's Topics:
1. why is ScopedTypeVariables not fixing this error? (Dennis Raddle)
2. Re: why is ScopedTypeVariables not fixing this error?
(Michael Snoyman)
3. Re: why is ScopedTypeVariables not fixing this error?
(Dennis Raddle)
----------------------------------------------------------------------
Message: 1
Date: Tue, 9 Oct 2018 01:45:18 -0700
From: Dennis Raddle <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: [Haskell-beginners] why is ScopedTypeVariables not fixing
this error?
Message-ID:
<CAKxLvoqky5U=5jgkbtjaavaf_f-ckgbmipktww_6xrgyh2q...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
In the following snippet from a program in progress (designed to compute
percentile rank for arbitrary lists of values) , I was hoping to declare
types of functions within the main function just as a way of helping myself
catch type errors. I'm getting the error "Can't match 'a' with 'a1'....
where 'a' is rigid type variable... etc. etc." on the line indicated in the
comment below. The usual error I get when I try to do this without
ScopedTypeVariables. So, I thought that ScopedTypeVariables was supposed to
allow this kind of usage. What am I doing wrong?
{-# LANGUAGE ScopedTypeVariables #-}
import qualified Data.Map as M
import qualified Data.List as L
import Data.Map(Map)
import Data.Function
-- <percent at or below> <percent below>
data PercentileData = PercentileData Double Double
-- new attempt, October 2018: using new PercentileData construct to
-- represent percentile in both ways. (at/below, or below)
computePercentile :: Ord a => Map a Double -> Map a PercentileData
computePercentile dataIn = error "foo"
where
pairs :: [(a,Double)] -- THIS IS THE LINE GETTING THE ERROR
pairs = L.sortBy (compare `on` snd) $ M.toList dataIn
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Message: 2
Date: Tue, 9 Oct 2018 11:48:56 +0300
From: Michael Snoyman <[email protected]>
To: Beginners <[email protected]>
Subject: Re: [Haskell-beginners] why is ScopedTypeVariables not fixing
this error?
Message-ID:
<CAKT9ecPqA=2BTk594SL5rHun=gl-0zjuthxy9zzcqohdboe...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
You need to put a `forall a.` in front of the `Ord a` constraint. To quote
the manual on the language extension[1]
> Enable lexical scoping of type variables explicitly introduced with forall
.
If it helps, the requirement of forall to be able to refer to the variable
was non-obvious to me the first time I tried to use the extension.
[1]
https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#ghc-flag--XScopedTypeVariables
On Tue, Oct 9, 2018 at 11:45 AM Dennis Raddle <[email protected]>
wrote:
> In the following snippet from a program in progress (designed to compute
> percentile rank for arbitrary lists of values) , I was hoping to declare
> types of functions within the main function just as a way of helping myself
> catch type errors. I'm getting the error "Can't match 'a' with 'a1'....
> where 'a' is rigid type variable... etc. etc." on the line indicated in the
> comment below. The usual error I get when I try to do this without
> ScopedTypeVariables. So, I thought that ScopedTypeVariables was supposed to
> allow this kind of usage. What am I doing wrong?
>
>
> {-# LANGUAGE ScopedTypeVariables #-}
>
> import qualified Data.Map as M
> import qualified Data.List as L
> import Data.Map(Map)
> import Data.Function
>
> -- <percent at or below> <percent below>
> data PercentileData = PercentileData Double Double
>
> -- new attempt, October 2018: using new PercentileData construct to
> -- represent percentile in both ways. (at/below, or below)
> computePercentile :: Ord a => Map a Double -> Map a PercentileData
> computePercentile dataIn = error "foo"
> where
> pairs :: [(a,Double)] -- THIS IS THE LINE GETTING THE ERROR
> pairs = L.sortBy (compare `on` snd) $ M.toList dataIn
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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Message: 3
Date: Tue, 9 Oct 2018 01:55:00 -0700
From: Dennis Raddle <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: Re: [Haskell-beginners] why is ScopedTypeVariables not fixing
this error?
Message-ID:
<CAKxLvorVN2Def+2QsZdDiH7ao-6f=ibj=drr3nrwdwq189w...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
That was easy. Thanks. I actually tried putting forall a in front of the
line getting the error, but to no avail. Now fixed.
D
On Tue, Oct 9, 2018 at 1:49 AM Michael Snoyman <[email protected]> wrote:
> You need to put a `forall a.` in front of the `Ord a` constraint. To quote
> the manual on the language extension[1]
>
> > Enable lexical scoping of type variables explicitly introduced with
> forall.
>
> If it helps, the requirement of forall to be able to refer to the variable
> was non-obvious to me the first time I tried to use the extension.
>
> [1]
> https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#ghc-flag--XScopedTypeVariables
>
> On Tue, Oct 9, 2018 at 11:45 AM Dennis Raddle <[email protected]>
> wrote:
>
>> In the following snippet from a program in progress (designed to compute
>> percentile rank for arbitrary lists of values) , I was hoping to declare
>> types of functions within the main function just as a way of helping myself
>> catch type errors. I'm getting the error "Can't match 'a' with 'a1'....
>> where 'a' is rigid type variable... etc. etc." on the line indicated in the
>> comment below. The usual error I get when I try to do this without
>> ScopedTypeVariables. So, I thought that ScopedTypeVariables was supposed to
>> allow this kind of usage. What am I doing wrong?
>>
>>
>> {-# LANGUAGE ScopedTypeVariables #-}
>>
>> import qualified Data.Map as M
>> import qualified Data.List as L
>> import Data.Map(Map)
>> import Data.Function
>>
>> -- <percent at or below> <percent below>
>> data PercentileData = PercentileData Double Double
>>
>> -- new attempt, October 2018: using new PercentileData construct to
>> -- represent percentile in both ways. (at/below, or below)
>> computePercentile :: Ord a => Map a Double -> Map a PercentileData
>> computePercentile dataIn = error "foo"
>> where
>> pairs :: [(a,Double)] -- THIS IS THE LINE GETTING THE ERROR
>> pairs = L.sortBy (compare `on` snd) $ M.toList dataIn
>>
>> _______________________________________________
>> 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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