Re: Haskell 1.3 (newtype)

[Simon L Peyton Jones <simonpj@dcs.gla.ac.uk>]

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From: Simon L Peyton Jones <simonpj@dcs.gla.ac.uk>
Subject: Re: Haskell 1.3 (newtype) 
Date: Thu, 14 Sep 1995 17:24:21 +0100
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Phil says:

| I think its vital that users know how to declare a new isomorphic
| datatype; it is not vital that they understand strictness declarations.
| Hence, I favor that
| 
| 	newtype Age = Age Int
| 	data Age = Age !Int
| 
| be synonyms, but that both syntaxes exist.
| 
| This is assuming I have understood Lennart correctly, and that
| 
| 	foo (Age n) = (n, Age (n+1))
|  	foo' a = (n, Age (n+1)) where (Age n) = a
| 
| are equivalent when Age is declared as a strict datatype. Unlike
| Sebastian or Simon, I believe it would be a disaster if for a newtype
| one had to distinguish these two definitions.

I agree that it is rather undesirable for them to differ.  If
someone had declared a *non-strict* verion like this:

	data Age = Age Int
 	foo (Age n) = (n, Age (n+1))

(where foo is patently non-strict in n), and then just wanted to say "do
away with the Age constructor", I'd like it to be a one-line change (data
--> newtype), rather than also having to add a twiddle to every
pattern-match:

 	foo ~(Age n) = (n, Age (n+1))

[which is eqiuvalent to using a where binding]

In effect, newtype could be explained as (a) a data decl with a !, and (b)
adding a ~ to every pattern match.

This is a hard one to call: which version actually requires least
explanation?!

Simon