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| author | David Feuer <david.feuer@gmail.com> | 2017-03-13 15:18:49 -0400 |
|---|---|---|
| committer | Ben Gamari <ben@smart-cactus.org> | 2017-03-13 15:18:56 -0400 |
| commit | dd3b06aa2d44fa160e36097ce90c4574160bd4eb (patch) | |
| tree | af71c83c0c0b203a00801fab0737368aa79ec0ad /compiler/stranal/DmdAnal.hs | |
| parent | 72ab738caf7539b12c88b7b0b23096d31cc0ab0f (diff) | |
| download | haskell-dd3b06aa2d44fa160e36097ce90c4574160bd4eb.tar.gz | |
Broaden demand analysis IO hack notes
The I/O hack for demand analysis has broader and arguably more
important implications than the note expressed. Broaden it.
[skip ci]
Reviewers: austin, bgamari
Reviewed By: bgamari
Subscribers: carter, rwbarton, thomie
Differential Revision: https://phabricator.haskell.org/D3307
Diffstat (limited to 'compiler/stranal/DmdAnal.hs')
| -rw-r--r-- | compiler/stranal/DmdAnal.hs | 12 |
1 files changed, 8 insertions, 4 deletions
diff --git a/compiler/stranal/DmdAnal.hs b/compiler/stranal/DmdAnal.hs index 9271eda8c5..25a4f8b912 100644 --- a/compiler/stranal/DmdAnal.hs +++ b/compiler/stranal/DmdAnal.hs @@ -344,10 +344,14 @@ dmdAnalAlt env dmd case_bndr (con,bndrs,rhs) {- Note [IO hack in the demand analyser] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ There's a hack here for I/O operations. Consider - case foo x s of { (# s, r #) -> y } -Is this strict in 'y'? Normally yes, but what if 'foo' is an I/O -operation that simply terminates the program (not in an erroneous way)? -In that case we should not evaluate 'y' before the call to 'foo'. + + case foo x s of { (# s', r #) -> y } + +Is this strict in 'y'? Often not! If foo x s performs some observable action +(including raising an exception with raiseIO#, modifying a mutable variable, or +even ending the program normally), then we must not force 'y' (which may fail +to terminate) until we have performed foo x s. + Hackish solution: spot the IO-like situation and add a virtual branch, as if we had case foo x s of |