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Diffstat (limited to 'docs/users_guide/profiling.rst')
| -rw-r--r-- | docs/users_guide/profiling.rst | 24 |
1 files changed, 18 insertions, 6 deletions
diff --git a/docs/users_guide/profiling.rst b/docs/users_guide/profiling.rst index b6db42ace1..c1d4d7d442 100644 --- a/docs/users_guide/profiling.rst +++ b/docs/users_guide/profiling.rst @@ -205,10 +205,22 @@ The syntax of a cost centre annotation for expressions is :: where ``"name"`` is an arbitrary string, that will become the name of your cost centre as it appears in the profiling output, and -``<expression>`` is any Haskell expression. An ``SCC`` annotation -extends as far to the right as possible when parsing. (SCC stands for -"Set Cost Centre"). The double quotes can be omitted if ``name`` is a -Haskell identifier, for example: :: +``<expression>`` is any Haskell expression. An ``SCC`` annotation extends as +far to the right as possible when parsing, having the same precedence as lambda +abstractions, let expressions, and conditionals. Additionally, an annotation +may not appear in a position where it would change the grouping of +subexpressions:: + + a = 1 / 2 / 2 -- accepted (a=0.25) + b = 1 / {-# SCC "name" #-} / 2 / 2 -- rejected (instead of b=1.0) + +This restriction is required to maintain the property that inserting a pragma, +just like inserting a comment, does not have unintended effects on the +semantics of the program, in accordance with `GHC Proposal #176 +<https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0176-scc-parsing.rst>`__. + +SCC stands for "Set Cost Centre". The double quotes can be omitted if ``name`` +is a Haskell identifier, for example: :: {-# SCC id #-} <expression> @@ -235,9 +247,9 @@ Here is an example of a program with a couple of SCCs: :: main = do let xs = [1..1000000] let ys = [1..2000000] print $ {-# SCC last_xs #-} last xs - print $ {-# SCC last_init_xs #-} last $ init xs + print $ {-# SCC last_init_xs #-} last (init xs) print $ {-# SCC last_ys #-} last ys - print $ {-# SCC last_init_ys #-} last $ init ys + print $ {-# SCC last_init_ys #-} last (init ys) which gives this profile when run: |