Does the Strict extension make monadic bindings strict?

[Johan Tibell]

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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> > ghc-devs at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> >
> >
> >
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