[Haskell-cafe] fromInteger for Lists

[andy morris]

2009/5/1 Paul Keir <pkeir at dcs.gla.ac.uk>:
> There's nothing better than making a data type an instance of Num. In
> particular, fromInteger is a joy. But how about lists?
>
> For example, if I have
>
> data Foo a = F [a]
>
> I can create a fromInteger such as
> fromInteger i = F [fromInteger i]
>
> and then a 19::(Foo Int), could become F [19].
>
> Is it possible to do something similar for lists? So could
> [1,2,3]::(Foo Int) become something slightly different, say,
>
> F [1,2,3]
>
> Paul
>
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>

If you mean what I think you're referring to, you can't. The only
reason it works for integer literals is that the compiler replaces
occurrences of, say, 19 with (fromInteger 19).

There's no function that's automatically applied to list literals, so
([1,2,3] :: Foo Int) isn't able to do anything useful, unfortunately.
However, there's an extension in GHC, OverloadedStrings, which lets
you use the method fromString of class Data.String.IsString to
overload literals. (That's not what you asked, though, I know. :) )