Does the Strict extension make monadic bindings strict?

8 Dec 2015

      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

Rob Stewart

David Kraeutmann

Ömer Sinan Ağacan

Johan Tibell

Adam Sandberg Eriksson

Rob Stewart

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