[Haskell-cafe] Newbie question on Haskell type

[Cale Gibbard]

There is a way to do this using a bunch of GHC extensions, but I get
the feeling that you're misinterpreting parametric polymorphism.
The type (Show a) => a -> Bool means that the function isString can be
implemented without caring what type a is, only knowing that it is in
Show.

As an example of this sort of thing, I know that there are only 4
values of type a -> Bool (without the class context). They are the
constant functions (\x -> True), (\x -> False), and two kinds of
failure (\x -> _|_), and _|_, where _|_ is pronounced "bottom" and
represents something along the lines of nontermination (aborting the
program also counts).

That said, typeclasses, along with undecidable, overlapping instances
(two GHC extensions) will allow us to do this, and without the Show
context, but with an additional context of our own, which every type
happens to be a member of:

class IsString t where
    isString :: t -> Bool

instance IsString [Char] where
    isString _ = True

instance IsString a where
    isString _ = False

Note that you need the command line options -fglasgow-exts,
-fallow-overlapping-instances, and -fallow-undecidable-instances in
order for this to work. Also, the types of things you pass to isString
have to be completely determined. Typing "isString 5" on the ghci
prompt will fail, because ghc will not be able to exactly tell which
type "5" is without context, which it needs to in order to determine
which instance of "isString" applies. Note that if you made String an
instance of Num, then 5 might really represent a string, and the
situation would be completely ambiguous. Typing isString (5 ::
Integer) will work, and return False.

Now here's another question: why do you want it? I can't imagine a
reasonable use of this function which wouldn't be better handled in
some other way. In fact, the only place I've seen this desire before
is in deciding whether to apply putStrLn or print to a value such that
no quotation marks are printed. In that case, you should seriously be
sure that you can't tell the difference between these statically.
Normally in code, where you are prepared to type putStrLn or print,
you know the type that's involved, and it only takes half a second to
think about which of the two to apply. Sometimes you genuinely want to
print a string rather than using putStrLn, as it converts nonprinting
characters into escapes, and clearly shows whether there are spaces at
the end of the string, for instance.
 - Cale

On 13/10/05, Huong Nguyen <hiperfume at gmail.com> wrote:
> Hi all,
>
> I want to write a small functionto test whether an input is a String or not.
> For example,
>
> isString::(Show a) =>a ->Bool
> This function will return True if the input is a string and return False if
> not
>
> Any of you have idea about that? Thanks in advance
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