-XStrict: Why some binders are not made strict?

[Roman Cheplyaka]

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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