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Today's Topics:
1. Non-exhaustive patterns (Galaxy Being)
2. Re: Non-exhaustive patterns (Pietro Grandinetti)
3. But I just want a function that returns stack depth!
(Michael Turner)
4. Re: But I just want a function that returns stack depth!
(יהושע ולך)
----------------------------------------------------------------------
Message: 1
Date: Sat, 11 Dec 2021 23:14:03 -0600
From: Galaxy Being <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Non-exhaustive patterns
Message-ID:
<cafahfsxzdqvh3fovvbjozxyso6pysjdeh1p9fod2pmz5mdu...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
This code
myTakePM :: Int -> [a] -> [a]
myTakePM 0 _ = []
myTakePM n (x:xs) = x : myTakePM (n-1) xs
is bad because it allows
myTakePM 4 [1,2,3]
[1,2,3*** Exception: <interactive>:(395,1)-(396,41): Non-exhaustive
patterns in function myTakePM
I knew it would not work, but why is it calling this essentially a partial
function? How does it know this? Again, I expected an error, but what is
this Non-exhaustive patterns in function myTakePM saying? Or, said another
way, what exactly is non-exhaustive about this?
--
⨽
Lawrence Bottorff
Grand Marais, MN, USA
[email protected]
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Message: 2
Date: Sun, 12 Dec 2021 05:21:34 +0000
From: Pietro Grandinetti <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Non-exhaustive patterns
Message-ID:
<pr2pr09mb3003c1974298e1fb4b6404e8fc...@pr2pr09mb3003.eurprd09.prod.outlook.com>
Content-Type: text/plain; charset="utf-8"
Hello - I think the pattern for the empty list (as second argument) is missing.
________________________________
From: Beginners <[email protected]> on behalf of Galaxy Being
<[email protected]>
Sent: Sunday, December 12, 2021 6:14:03 AM
To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level
topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Non-exhaustive patterns
This code
myTakePM :: Int -> [a] -> [a]
myTakePM 0 _ = []
myTakePM n (x:xs) = x : myTakePM (n-1) xs
is bad because it allows
myTakePM 4 [1,2,3]
[1,2,3*** Exception: <interactive>:(395,1)-(396,41): Non-exhaustive patterns
in function myTakePM
I knew it would not work, but why is it calling this essentially a partial
function? How does it know this? Again, I expected an error, but what is this
Non-exhaustive patterns in function myTakePM saying? Or, said another way, what
exactly is non-exhaustive about this?
--
⨽
Lawrence Bottorff
Grand Marais, MN, USA
[email protected]<mailto:[email protected]>
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------------------------------
Message: 3
Date: Sun, 12 Dec 2021 19:57:03 +0900
From: Michael Turner <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] But I just want a function that returns
stack depth!
Message-ID:
<canhebiiigcbhu9wbcqtjwypnw9btesnzsxfbp_+rmewdmhs...@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
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
[email protected]
Understand - http://www.projectpersephone.org/
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"Love does not consist in gazing at each other, but in looking outward
together in the same direction." -- Antoine de Saint-Exupéry
------------------------------
Message: 4
Date: Sun, 12 Dec 2021 13:46:28 +0200
From: יהושע ולך <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] But I just want a function that
returns stack depth!
Message-ID:
<CAMcanRWt9_qJk5C=3y1z5pbfx7gjngmfta4u18smfcj75g0...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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>
You need
```
let l = getCallStack callStack
print (length l)
```
the `getCallStack` does not return an "IO" value..
On Sun, Dec 12, 2021, 12:58 Michael Turner <[email protected]>
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
> [email protected]
>
> 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
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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