Send Beginners mailing list submissions to
beginners@haskell.org
To subscribe or unsubscribe via the World Wide Web, visit
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
or, via email, send a message with subject or body 'help' to
beginners-requ...@haskell.org
You can reach the person managing the list at
beginners-ow...@haskell.org
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Beginners digest..."
Today's Topics:
1. Understanding types and constructors more thoroughly
(Josh Diamond)
2. Re: Understanding types and constructors more thoroughly
(Josh Diamond)
----------------------------------------------------------------------
Message: 1
Date: Thu, 21 May 2020 10:45:50 +0000
From: Josh Diamond <evild...@live.co.uk>
To: "beginners@haskell.org" <beginners@haskell.org>
Subject: [Haskell-beginners] Understanding types and constructors more
thoroughly
Message-ID:
<byapr05mb4215683ba1af3b26d33e13f3f8...@byapr05mb4215.namprd05.prod.outlook.com>
Content-Type: text/plain; charset="utf-8"
Ok so I have an assessment, first part is:
Consider the following declaration for a parameterised type of lists in which
Nil represents the empty list, and Cons x xs represent a non-empty list with
first element x and remaining list of elements xs:
data List a = Nil | Cons a (List a)
Am I right in reading it like: Declare a List type which can take any parameter
and is constructed by Nil or Cons which itself takes anything and a List of
anything?
Also it seems strange to define a List without [] or : like in the prelude but
I assume it’s just for the purposes of the question. Is the (List a) used just
to make the order of evaluation explicit or is it some weird thing with tuples?
Ok so the actual question is:
Q) Define a recursive function append :: List a -> List a -> List a that
appends two lists together to give a single list.
So I’d have to do something similar to how append is done in the standard
prelude I.e.
(++) :: [a] -> [a] -> [a]
[] ++ ys = ys
(x:xs) ++ ys = x : (xs ++ ys)
How do I do pattern matching on a user defined type?
I was thinking something like:
Nil append List a = List a
For the first line but the second line is confusing me. I’ve read some stuff on
pattern matching for data constructors but I don’t know how relevant it is here.
Maybe something like this would work:
Cons a (List a) append List b = a : (List a append List b)
But I’m getting ‘Data constructor not in scope’ for everything to do with
List...it thinks it’s a data constructor although I’ve defined it as a type
above.
Any help would be great 😊
Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10
Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://mail.haskell.org/pipermail/beginners/attachments/20200521/d625cf54/attachment-0001.html>
------------------------------
Message: 2
Date: Thu, 21 May 2020 11:09:16 +0000
From: Josh Diamond <evild...@live.co.uk>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Understanding types and constructors
more thoroughly
Message-ID:
<byapr05mb42154bb12dd1284ee750deeff8...@byapr05mb4215.namprd05.prod.outlook.com>
Content-Type: text/plain; charset="utf-8"
Actually I worked it out, no need for any help so please ignore
Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10
From: Josh Diamond<mailto:evild...@live.co.uk>
Sent: 21 May 2020 11:47
To: beginners@haskell.org<mailto:beginners@haskell.org>
Subject: [Haskell-beginners] Understanding types and constructors more
thoroughly
Ok so I have an assessment, first part is:
Consider the following declaration for a parameterised type of lists in which
Nil represents the empty list, and Cons x xs represent a non-empty list with
first element x and remaining list of elements xs:
data List a = Nil | Cons a (List a)
Am I right in reading it like: Declare a List type which can take any parameter
and is constructed by Nil or Cons which itself takes anything and a List of
anything?
Also it seems strange to define a List without [] or : like in the prelude but
I assume it’s just for the purposes of the question. Is the (List a) used just
to make the order of evaluation explicit or is it some weird thing with tuples?
Ok so the actual question is:
Q) Define a recursive function append :: List a -> List a -> List a that
appends two lists together to give a single list.
So I’d have to do something similar to how append is done in the standard
prelude I.e.
(++) :: [a] -> [a] -> [a]
[] ++ ys = ys
(x:xs) ++ ys = x : (xs ++ ys)
How do I do pattern matching on a user defined type?
I was thinking something like:
Nil append List a = List a
For the first line but the second line is confusing me. I’ve read some stuff on
pattern matching for data constructors but I don’t know how relevant it is here.
Maybe something like this would work:
Cons a (List a) append List b = a : (List a append List b)
But I’m getting ‘Data constructor not in scope’ for everything to do with
List...it thinks it’s a data constructor although I’ve defined it as a type
above.
Any help would be great 😊
Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10
Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://mail.haskell.org/pipermail/beginners/attachments/20200521/10cead63/attachment.html>
------------------------------
Subject: Digest Footer
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
------------------------------
End of Beginners Digest, Vol 143, Issue 9
*****************************************
