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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