Re: Does the Strict extension make monadic bindings strict?

In which case, I've created a ticket to record this bug and to track its fix: https://ghc.haskell.org/trac/ghc/ticket/11193 On 10 December 2015 at 15:26, Adam Sandberg Eriksson < adam@sandbergericsson.se> wrote:

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 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 :

...

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#stric...

...

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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