Re: Readline read_history and write_history addition

On Jan 21, 2008 10:27 AM, Judah Jacobson wrote:

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On Jan 19, 2008 11:09 AM, Judah Jacobson wrote:

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On Jan 18, 2008 11:10 PM, Alexander Dunlap wrote:

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Hello all,

I'd like to propose the addition of read_history and write_history bindings to the readline library. I believe I've followed the instructions on the Haskell wiki appropriately and I've set up a ticket (with a patch) at http://hackage.haskell.org/trac/ghc/ticket/2053.

Essentially, the patch adds simple bindings so that Haskell programs can use Readline's read_history and write_history functions. This will be useful for the implementation of http://hackage.haskell.org/trac/ghc/ticket/2050 (add persistent history to GHCi) and potentially other applications.

Thanks for your time and consideration.

Alex

That patch looks good to me. While we're at it, can we also add the following related functions, which are used to control the maximum size of the history file? (They're also provided by editline.)

void clear_history(void); void stifle_history(int); int unstifle_history(void); int history_is_stifled(void);

One more suggestion, from Robert Dockins (author of the Shellac and Shellac-readline packages):

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The only concern I have is that this patch doesn't seem to be handling errors properly. read_history and write_history should return errno, but this binding has them returning (). These functions do file operations and therefore can fail; we want (be able) to know when that happens.

I think we should just throw an error if those functions return a nonzero value; for example, we already do that in the functions readInitFile and parseAndBind.

Thanks, -Judah

I'm reluctant to use the throw an error solution because these functions failing does not have to be the end of the world (or even necessarily handled by the application). If the history file can't be found, the user just doesn't get their history restored (in fact, this may not even be a problem: if the user hasn't used the application before, readHistory will fail silently on the first run and then work fine after the history has been saved at the end of the first session). Similarly, writing or appending to the history file is generally not an essential task and can fail without terminating the program. (I know there's catch, but I don't think the programmer even has to worry about it that much.) I think that just returning a value for success and a value for failure would be appropriate. However, I'm not sure how we would implement it without using error. The usual Haskell solution would be to use Maybe, but what would we have Just of, since the functions don't return real values? Is Just () an accepted idiom? (I've never seen it, but I haven't seen all the Haskell there is to see, either.) Thanks. Alex