Does the Strict extension make monadic bindings strict?

[Adam Sandberg Eriksson]

I agree that this seems to be a bug. I have a lot to do currently, but
might be able to look at it sometime during next week.

Adam Sandberg Eriksson


On Thu, 10 Dec 2015, at 03:34 PM, Johan Tibell wrote:
> I believe this is just a bug, since the desugaring ought to be strict
> in the \x.
>
> On Tue, Dec 8, 2015 at 6:35 PM, Ömer Sinan Ağacan
> <omeragacan at gmail.com> wrote:
>> I think this is a problem/bug in the implementation. In the "function
>>
definitions" section of the wiki page it says the argument will have a
>>
bang pattern. But then this code:
>>
>>
do x <- ...
>>
return (x + 1)
>>
>>
which is just a syntactic sugar for `... >>= \x -> return (x + 1)`
>>
doesn't have the bang pattern in `x`.
>>
>>
(See also a related email I sent to ghc-devs yesterday:
>> https://mail.haskell.org/pipermail/ghc-devs/2015-December/010699.html)
>>
>>
2015-12-08 12:27 GMT-05:00 David Kraeutmann <kane at kane.cx>:
>>
> While there's a fundamental difference between (>>=) and let-
> bindings, it
>>
> might be worth adding to the docs that -XStrict only makes let
> bindings
>>
> strict.
>>
>
>>
>
>>
> On 12/08/2015 06:22 PM, Rob Stewart wrote:
>>
>
>>
> Are the following two programs equivalent with respect to the
> strictness
>>
> of `readFile`?
>>
>
>>
> --8<---------------cut here---------------start------------->8---
>>
> {-# LANGUAGE BangPatterns #-}
>>
>
>>
> module Main where
>>
>
>>
> main = do
>>
>!contents <- readFile "foo.txt"
>>
>print contents
>>
> --8<---------------cut here---------------end--------------->8---
>>
>
>>
> And:
>>
>
>>
> --8<---------------cut here---------------start------------->8---
>>
> {-# LANGAUGE Strict #-}
>>
>
>>
> module Main where
>>
>
>>
> main = do
>>
>contents <- readFile "foo.txt"
>>
>print contents
>>
> --8<---------------cut here---------------end--------------->8---
>>
>
>>
> The documentation on "Strict-by-default pattern bindings" gives
>>
> let/where binding as an example, but there is not a monadic bind
> example.
>>
> http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html#strict-by-default-pattern-bindings
>>
>
>>
> Inspecting GHC Core for these two programs suggests that
>>
>
>>
> !contents <- readFile "foo.txt"
>>
>
>>
> is not equivalent to (with Strict enabled):
>>
>
>>
> contents <- readFile "foo.txt"
>>
>
>>
> Here's core using BangPatterns:
>>
>
>>
> (readFile (unpackCString# "foo.txt"#))
>>
> (\ (contents_asg :: String) ->
>>
>case contents_asg of contents1_Xsk { __DEFAULT ->
>>
>print @ String $dShow_rYy contents1_Xsk
>>
>})
>>
>
>>
> Here's core using Strict:
>>
>
>>
> (readFile (unpackCString# "foo.txt"#))
>>
> (\ (contents_asg :: String) ->
>>
>print @ String $dShow_rYv contents_asg)
>>
>
>>
> Does this core align with the design of the Strict extension?
>>
>
>>
> If it does, are users going to understand that using Strict is
> going to
>>
> make let/where bindings strict, but is not going to make <- or >>=
>>
> bindings strict?
>>
>
>>
> --
>>
> Rob Stewart
>>
>
>>
>
>>
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>>
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>>
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>>
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>>
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