GHC deliberately rejects programs with unreachable code, as Ashley says,
because
a) they often are programmer errors
b) I can't think of a time when they'd be useful.
I don't see what's wrong with
neverT :: T Char -> x
neverT = error "urk"
After all, since you can't construct a value of type (T Char) there's
not much point in pattern-matching on it.
Simon
| -----Original Message-----
| From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
On Behalf Of Lennart
| Augustsson
| Sent: 08 January 2005 10:30
| To: Ashley Yakeley
| Cc: [email protected]
| Subject: Re: [Haskell] Re: "Never" GADT Function?
|
| Well, this compiles:
|
| data T a where
| BoolT :: T Bool
| IntT :: T Int
|
| neverT' :: T a -> x
| neverT' BoolT = error "Bool"
| neverT' IntT = error "Int"
|
| neverT :: T Char -> x
| neverT = neverT'
|
| But it uses error for the unreachable cases, maybe not what you want.
|
| -- Lennart
|
| Ashley Yakeley wrote:
| > In article <[EMAIL PROTECTED]>,
| > Martin Sulzmann <[EMAIL PROTECTED]> wrote:
| >
| > You should be able to even write
| >
| > neverT (BoolT x) = x
| > neverT (IntT x) = False
| >
| > Actually I didn't put in any arguments to my constructors. Apart
from
| > that I agree: this should compile, but doesn't:
| >
| > data T a where
| > BoolT :: T Bool
| > IntT :: T Int
| >
| > neverT :: T Char -> x
| > neverT BoolT = "hello"
| > neverT IntT = 37
| >
| > Pick.hs:11:9:
| > Inaccessible case alternative: Can't match types `Bool' and
`Char'
| > When checking the pattern: BoolT
| > In the definition of `neverT': neverT BoolT = "hello"
| >
|
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