-XStrict: Why some binders are not made strict?

Thu Dec 10 14:34:03 UTC 2015

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?

On Tue, Dec 8, 2015 at 3:42 PM, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> Adam, Johan,
>
> Looking at the user manual
>
> http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html#strict-haskell
> ,
> and indeed the wiki page
> https://ghc.haskell.org/trac/ghc/wiki/StrictPragma
> it's not really clear whether the sub-components of a pattern are strict.
> That is, is the second equation of zip strict in x, and xs?   (Supposing
> for now that the list data structure is lazy).  The manual doesn't say one
> way or the other.
>
> What's the answer?  And could the user manual please say?
>
> Thanks
>
> Simon
>
> | -----Original Message-----
> | From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Ömer
> | Sinan Agacan
> | Sent: 08 December 2015 01:41
> | To: ghc-devs <ghc-devs at haskell.org>
> | Subject: -XStrict: Why some binders are not made strict?
> |
> | Let's say I have this code:
> |
> |     zip :: [a] -> [b] -> [(a, b)]
> |     zip [] [] = []
> |     zip (x : xs) (y : ys) = (x, y) : zip xs ys
> |
> | With -XStrict 'x', 'xs', 'y' and 'ys' don't become strict. I'm wondering
> | about
> | the motivation behind this, I found this interesting. I always thought -
> | XStrict
> | gives me this guarantee: If I'm using an already-defined variable(bound
> by
> | a
> | let or pattern matching) in an expression, I can be sure that the
> variable
> | won't be bottom in that expression, because it would be `seq`d before the
> | expression is evaluated.
> |
> | So if I have
> |
> |     case ... of
> |         D x y -> <body>
> |
> | or
> |
> |     let x = ...
> |         y = ...
> |      in <body>
> |
> | In both cases I was thinking that in <body> 'x' and 'y' can't be
> | bottom(with
> | -XStrict). This would make -XStrict programs evaluate like they would in
> a
> | call-by-value language(even though in the RTS level thunks will be
> built).
> | Variables can't range over computations; all binders evaluated strictly
> | etc.
> |
> | Can anyone tell me about the motivation behind this decision?
> |
> | I think the wiki page actually conflicts with itself. It says "...
> | bindings to be
> | strict by default" but then in "case expressions" sections says
> |
> |     case x of (a,b) -> rhs
> |
> |     is interpreted as
> |
> |     case x of !(a,b) -> rhs
> |
> | Here bindings 'a' and 'b' are not made strict. I'd expect something like:
> |
> |     case x of (!a,!b) -> rhs
> |
> | (Which seems to be invalid Haskell, but same effect could be achieved
> with
> | `seq
> | a (seq b rhs)`)
> |
> | Thanks..
> |
> | (I also realized that the wiki page doesn't mention bindings in do
> syntax,
> | is
> | it because this case is implied by "function definitions"? That is, bangs
> | are
> | added after do syntax is desugared and so they become strict?)
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