On 12/10/2015 04:34 PM, Johan Tibell wrote: > I'm snowed under but I promise I will try to reply soon! To think about > in the mean time: what do existing strict languages with pattern > matching do?
Well, strict languages do not have lazy data to force to begin with, do they? Personally, I find the simple intuition of "all patterns are strict by default" rather appealing. E.g. I wouldn't expect the expressions let (v1,v2) = a in f v2 and let (v1,v2) = a; v3 = v2 in f v3 to have different semantics. If we decide to adopt this semantics, we need to address the meaning of the pattern ~(v1, v2) under -XStrict. Intuitively, ~ should propagate to the subpatterns. An alternative is to disallow this pattern under -XStrict and require writing all ~s explicitly, which may get tedious: ~(~v1, ~v2) ~(~v1, ~(~v2, ~v3)) etc. We also need to ensure the consistency between this extension and the unlifted data types proposal [1], given their similarity. Interestingly, I don't see constructor patterns explained there either. [1]: https://ghc.haskell.org/trac/ghc/wiki/UnliftedDataTypes#Dynamicsemanticsofunliftedtypes
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