The important thing here, is that print is a function from a "pure" value (any type implements the Show typeclass. (similar to trait in rust, or interface in some languages))

and creates a "IO" value, which can be thought of as a IO action. inside the IO monad, that just means running it.

‪On Sun, Dec 12, 2021 at 7:05 PM ‫יהושע ולך‬‎ <yehoshuapw@gmail.com> wrote:‬

Notice: (from ghci)

Prelude GHC.Stack> :t callStack

callStack :: CallStack
Prelude GHC.Stack> :t getCallStack
getCallStack :: CallStack -> [([Char], SrcLoc)]
Prelude GHC.Stack> :t getCallStack callStack
getCallStack callStack :: [([Char], SrcLoc)]
Prelude GHC.Stack> :t length (getCallStack callStack)

length (getCallStack callStack) :: Int

Prelude GHC.Stack> :t print (length (getCallStack callStack))

print (length (getCallStack callStack)) :: IO ()

Prelude GHC.Stack> :t print

print :: Show a => a -> IO ()

the `print` function takes a pure value, and creates an IO value from it.

(actually printing is an IO action. the value it prints is not)

On Sun, Dec 12, 2021 at 5:32 PM Michael Turner <michael.eugene.turner@gmail.com> wrote:

Regards,
Michael Turner
Executive Director
Project Persephone
1-25-33 Takadanobaba

Shinjuku-ku Tokyo 169-0075
Mobile: +81 (90) 5203-8682
turner@projectpersephone.org

Understand - http://www.projectpersephone.org/
Join - http://www.facebook.com/groups/ProjectPersephone/
Donate - http://www.patreon.com/ProjectPersephone
Volunteer - https://github.com/ProjectPersephone

"Love does not consist in gazing at each other, but in looking outward
together in the same direction." -- Antoine de Saint-Exupéry




On Sun, Dec 12, 2021 at 8:48 PM <beginners-request@haskell.org> wrote:


> have another look at
> https://hackage.haskell.org/packaggetCallStackgetCallStacke/base-4.16.0.0/docs/GHC-Stack.html#v:callStack
> <https://hackage.haskell.org/package/base-4.16.0.0/docs/GHC-Stack.html#v:callStack>

I'd been looking at that already, over and over. Still not seeing it.
But your proposed fix does work. Thank you.
> let l = getCallStack callStack
> print (length l)
> ```
> the `getCallStack` does not return an "IO" value..

And yet it's it's somehow an IO value in "print (length (getCallStack
callStack))"?

I just don't get this.

> On Sun, Dec 12, 2021, 12:58 Michael Turner <michael.eugene.turner@gmail.com>
> wrote:
>
> > The following prints 1, as you'd expect:
> > -----------------
> > import GHC.Stack
> >
> > foo :: HasCallStack => IO ()
> > foo = do
> >    print (length (getCallStack callStack))
> >
> > main =
> >   foo
> > -------------------
> > But when I make it this:
> > ...
> >    l <- getCallStack callStack ;
> >    print (length l)
> >
> >  I get all this:
> > ------------------------------------------
> > ...
> >     • Couldn't match type ‘[]’ with ‘IO’
> >       Expected type: IO ([Char], SrcLoc)
> >         Actual type: [([Char], SrcLoc)]
> >     • In a stmt of a 'do' block: l <- getCallStack callStack
> >       In the expression:
> >         do l <- getCallStack callStack
> >            print (length l)
> >       In an equation for ‘foo’:
> >           foo
> >             = do l <- getCallStack callStack
> >                  print (length l)
> >   |
> > 5 |    l <- getCallStack callStack ;
> >   |         ^^^^^^^^^^^^^^^^^^^^^^
> > --------------------------------------------
> >
> > What am I not seeing?
> >
> >
> > Regards,
> > Michael Turner
> > Executive Director
> > Project Persephone
> > 1-25-33 Takadanobaba
> >
> > Shinjuku-ku Tokyo 169-0075
> > Mobile: +81 (90) 5203-8682
> > turner@projectpersephone.org
> >
> > Understand - http://www.projectpersephone.org/
> > Join - http://www.facebook.com/groups/ProjectPersephone/
> > Donate - http://www.patreon.com/ProjectPersephone
> > Volunteer - https://github.com/ProjectPersephone
> >
> > "Love does not consist in gazing at each other, but in looking outward
> > together in the same direction." -- Antoine de Saint-Exupéry
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