[Haskell-beginners] How to get qualifiers into type

[Rustom Mody]

On Thu, Jan 17, 2013 at 9:02 PM, Kim-Ee Yeoh <ky3 at atamo.com> wrote:

> On Thu, Jan 17, 2013 at 8:40 PM, Rustom Mody <rustompmody at gmail.com>wrote:
>
>> And I need to do things like
>>
>> type Vertex = Ix a => a
>> (so that a vertex can be used to index an adjacency-list-array)
>>
>
> Two things seem to be going on:
>
> (1) you're trying to define a data type using "type" and not "data"+class
> instances. Recall that the "type" declares a type /synonym/, and while type
> synonyms seem to behave like CPP-style macros, they aren't.
>
>
>> That is to say that whenever 'Vertex' appears in a type signature, the Ix
>> should 'float' out to the qualifier list
>>
>
> (2) you're trying to save on keyboarding by using Vertex as a CPP-style
> macro that expands in the "smart" way you just described. Something that
> has been requested before is collapsing a list of constraints into just one
> (search haskell-cafe archives and also see "No Module Abstraction" of [1]).
> The larger the number of constraints, i.e. (A x) expands out to (B x, C x,
> D x, ...) the greater the need for such a feature. But there's no benefit
> to be gained here.
>
> (3) you're trying to improve readability of code because "Ix a => a" isn't
> explicit that type variable "a" is a vertex. Nothing stops you from writing
> "Ix vertex => vertex" however. See [2].
>
> [1] http://lukepalmer.wordpress.com/2010/03/19/haskells-big-three/
>

Thanks for that link. So it seems its not possible to do what I want.

my aesthetic choice: I don’t obscure my code’s inner beauty behind
> repulsive indecipherable type signatures…
>

Vow! Thats stronger language than I am habituated to use and yes thats what
I am talking about.

And further if the type signature is already turgid and obese with
super-generic constraints, writing (... Ix vertex...) => ... instead of Ix
v just worsens it



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