Re: [Haskell-cafe] Data & newtype differences. Today: strictness

Thanks, that's clearer to me now. It confirmed my thoughts:

Matching the pattern con pat against a value, where con is a constructor defined by newtype, depends on the value: - If the value is of the form con v, then pat is matched against v. - If the value is ⊥, then pat is matched against ⊥.

Put shorter: newtype TestN = TestN Int TestN x = undefined Then x = undefined. Which means you cannot make a boxed type out of un unboxed one using newtype. That makes sense. 2012/1/22 Roman Cheplyaka

* Yves Parès [2012-01-22 11:32:30+0100]

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These make me think that pattern matching against a newtype is always lazy (irrefutable). Am I right?

Yes.

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Is there some litterature expliciting in a less empiric way than I did the differences like this between data and newtype? I've never come against such documentation through all my learning of Haskell, yet I think it's an important point.

See the Haskell report, section 3.17.2 "Informal Semantics of Pattern Matching" [1].

In particular, this paragraph:

The irrefutable patterns are as follows: a variable, a wildcard, N apat where N is a constructor defined by newtype and apat is irrefutable (see Section 4.2.3), var@apat where apat is irrefutable, or of the form ~apat (whether or not apat is irrefutable). All other patterns are refutable.

[1]: http://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-600003.17...

-- Roman I. Cheplyaka :: http://ro-che.info/