[Haskell-cafe] Re: Handles with their IOMode in their type
gmane at asztal.net
Tue Dec 8 13:22:41 EST 2009
On 08/12/2009 03:54, Bas van Dijk wrote:
> Could not get to sleep tonight so I got up and hacked this together:
> Does something like this already exist on hackage:
> If not, I may turn this into a package and upload it tomorrow.
> Comments, criticism and patches are welcome.
I like this idea.
A small observation, though:
> stdin :: ReadModes ioMode => Handle ioMode
> stdin = Handle SIO.stdin
This allows writing to stdin by choosing ReadWriteMode as the ioMode. I think it would be better to have just
> stdin :: Handle ReadMode
*HandleExplicitIOMode> hPutStrLn (stdin :: Handle ReadWriteMode) "This shouldn't typecheck!"
*** Exception: <stdin>: hPutStr: illegal operation (handle is not open for writing)
This also shows another reason for stdin, stdout and stderr to be monomorphic:
> hGetLine stdin
Ambiguous type variable `ioMode' in the constraint:
arising from a use of `hGetLine' at <interactive>:1:0-13
Probable fix: add a type signature that fixes these type variable(s)
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