On 9/17/07, Henning Thielemann wrote:

On Sat, 15 Sep 2007, Sam Hughes wrote:

...

Prelude> (x,y) <- return $ (repeat 1, repeat 2)

You didn't tell, which Monad this shall be.

GHCi always runs in the IO monad. -- ryan

| It seems that GHCi outputs the contents of the variable you've created | when there's only one of them.

Indeed, that is documented behaviour (first bullet here: http://www.haskell.org/ghc/docs/latest/html/users_guide/ch03s04.html#ghci-st... ) Perhaps it's confusing behaviour? If so do suggest an alternative.

why not simply do what the flag suggests, either always try to print the bind-result, or never? i assume there has been a use case where this special case has been found useful/necessary? as for earlier questions/suggestions in this thread:

...

Is there a way to make GHCi not print the result of an action but still make my variables get bound?

This seems to be a common question (I myself asked it recently), so I've added an entry to the GHCi page on the wiki.

Ideally, it would be nice if this were discoverable from within GHCi, but I'm not sure how this would best be done.

i've run into this myself!-) there is a patch pending for ghc head (so it may not be in 6.8.1:-( that would show the GHCi-specific flags together with the GHCi options: Prelude> :set options currently set: none. GHCi-specific dynamic flag settings: -fno-print-explicit-foralls -fprint-bind-result -fno-break-on-exception -fno-break-on-error -fno-print-evld-with-show other dynamic, non-language, flag settings: .. so you'd at least know which possibly relevant flags there are, and if the names are not expressive enough, the :help would point you to the flag reference: :help .. Options for ':set' and ':unset': +r revert top-level expressions after each evaluation +s print timing/memory stats after each evaluation +t print type after evaluation -<flags> most GHC command line flags can also be set here (eg. -v2, -fglasgow-exts, etc.) for GHCi-specific flags, see User's Guide, Flag reference, Interactive-mode options ..

I've always wondered if ghc(i) --help should be a bit more instructive, or perhaps if there were a man page that lay somewhere between the --help message and the manual in terms of comprehensiveness. It's a pretty major jump from a short description of 4 command line options (only one of which I have ever used) to the entire manual, with a ~10 page table of contents.

please note that the flag reference, which was pointed to earlier in this thread, is just such a summary, and there's a subsection dedicated to GHCi-specific options. there isn't much in that section in 6.6.1: http://www.haskell.org/ghc/docs/latest/html/users_guide/flag-reference.html#... but in ghc head and the upcoming 6.8, you'll find most of the GHCi-related flags summarized there. hth, claus

On 9/20/07, Simon Peyton-Jones wrote:

Hmm. Currently GHCi prints the value of the *variable* that you bind. For example

Just x <- foo 4

will print the value of x, *not* the value of (Just x). So if two variables are bound, there'd be two values to print.

But an alternative behaviour would be to print the value of the LHS, constructors and all. That is in the above example, print Just <value of x> Then if there were two values, for example (x,Just y) <- bar 5 then it'd print ( <value of x>, Just <value of y> )

That would be a change from current behaviour, but it'd look identical when the LHS was a simple variable (the wildly common case), and it'd do something useful when multiple variables are bound.

I think a more consistent behavior would be to not print the LHS at all. If you wanted to print the result of the computation you could just do: Prelude> bar 5 or, if you also wanted bound variables afterwards: Prelude> (x, Just y) <- bar 5 Prelude> (x, Just y) Perhaps this is my imperative background speaking, but I don't see much difference at the GHCi prompt between these two: Prelude> let x = 5 Prelude> x <- return 5 I think they should have the same behavior by default; otherwise it's just confusing to the new user. For reference, I ran into this problem while playing around on the ICFP contest problem, and I became quite frustrated trying to do dna <- Data.ByteString.Char8.readFile "endo.dna" and having to wait a few minutes for the console to catch up while using GHCi as a simple interactive debugger. The best argument I can think of for changing the default value of that option is this: The workaround for printing the value of bound variables, if the default is to not print and you don't know the options, is simple: just type the bound variable in as an expression afterwards. On the other hand, there is no workaround to not print the value of bound variables, if you don't know about the existence of that option. In my case, I did do a cursory look to find an option to change this behavior, but I missed it in the documentation. -- ryan

| I think a more consistent behavior would be to not print the LHS at | all. If you wanted to print the result of the computation you could | just do: | | Prelude> bar 5 | | or, if you also wanted bound variables afterwards: | | Prelude> (x, Just y) <- bar 5 | Prelude> (x, Just y) I've added Ryan's comments to http://hackage.haskell.org/trac/ghc/ticket/1721 Others might want to add further suggestions there. Simon

I've always wondered if ghc(i) --help should be a bit more instructive, or perhaps if there were a man page that lay somewhere between the --help message and the manual in terms of comprehensiveness. It's a pretty major jump from a short description of 4 command line options (only one of which I have ever used) to the entire manual, with a ~10 page table of contents. On 16/09/2007, Stuart Cook wrote:

On 9/16/07, Ryan Ingram wrote:

...

Is there a way to make GHCi not print the result of an action but still make my variables get bound?

This seems to be a common question (I myself asked it recently), so I've added an entry to the GHCi page on the wiki.

Ideally, it would be nice if this were discoverable from within GHCi, but I'm not sure how this would best be done.

Stuart _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

On 9/17/07, Martin Lütke wrote:

What is the url for the wiki entry?

There was already a page at http://haskell.org/haskellwiki/GHC/GHCi so I put it there, but I also took the liberty of creating some #REDIRECTs, so http://haskell.org/haskellwiki/ghci should work just as well. Stuart