Re: [Haskell-cafe] Why isn't pattern matching lazy by default?

Chaddaï Fouché wrote:

2007/9/19, Miguel Mitrofanov :

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Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. It's even more confusing that pattern matching in 'let' _is_ lazy. No, it's not.

See, in let or where constructs you don't have a choice; you can't do different things depending on whether some value is Just x or Nothing. Therefore, there is no need to perform pattern matching strictly.

In other words, the pattern matching in case can't be lazy by default since the primary purpose of case is to choose an alternative and for that you NEED to evaluate the matched value. In let or where, the pattern matching is only there to allow convenient deconstruction of the value, but you don't make any choice based on the form of the value, so you can afford to be lazy (which is the default in the language everywhere it makes sense, it doesn't in a normal "case" construct). If you want to use your pattern matching in a case only to deconstruct the value but not to choose an alternative, you need to specify it using ~ .

So, IMHO, the rules on strictness of the pattern matching are perfectly consistent and clear. You just need to reflect on the purpose of the constructs where you use it..

Except that newtype deconstruction doesn't introduce strictness - and you can't necessarily see from the syntax of a pattern-match whether it's newtype or data. Isaac