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First cut at documentation for HPC option in GHC

parent 5a1a0eb0
......@@ -1188,6 +1188,39 @@
<title>Program coverage options</title>
<para><xref linkend="hpc"/></para>
<tgroup cols="4" align="left" colsep="1" rowsep="1">
<entry>Turn on Haskell program coverage instrumentation</entry>
<entry><option>-hpcdir dir</option></entry>
<entry>Directory to deposit .mix files during compilation</entry>
<title>Haskell pre-processor options</title>
......@@ -1745,6 +1778,12 @@
<entry>Dump after intrumentation for program coverage</entry>
<entry>Dump inlining info</entry>
......@@ -1302,8 +1302,128 @@ to re-read its input file:
<sect1 id="hpc">
<title>Observing Code Coverage</title>
<indexterm><primary>code coverage</primary></indexterm>
<indexterm><primary>Haskell Program Coverage</primary></indexterm>
Code coverage tools allow a programer to determine what parts of
their code have been actually executed, and which parts have
never actually been invoked. GHC has an option for generating
instrumented code that records code coverage as part of the
<ulink url="">Haskell Program Coverage
</ulink>(HPC) toolkit. HPC tools can be used to render the
outputed code coverage infomation into human understandable
HPC provides coverage information of two kinds: source coverage
and boolean-control coverage. Source coverage is the extent to
which every part of the program was used, measured at three
different levels: declarations (both top-level and local),
alternatives (among several equations or case branches) and
expressions (at every level). Boolean coverage is the extent to
which each of the values True and False is obtained in every
syntactic boolean context (ie. guard, condition, qualifier).
HPC displays both kinds of information in two different ways:
textual reports with summary statistics (hpc-report) and sources
with color mark-up (hpc-markup). For boolean coverage, there
are four possible outcomes for each guard, condition or
qualifier: both True and False values occur; only True; only
False; never evaluated. In hpc-markup output, highlighting with
a yellow background indicates a part of the program that was
never evaluated; a green background indicates an always-True
expression and a red background indicates an always-False one.
<sect2><title>A small example: Reciprocation</title>
For an example we have a program which computes exact decimal
representations of reciprocals, with recurring parts indicated in
brackets. We first build an instrumented version using the
hpc-build script. Assuming the source file is Recip.hs.
reciprocal :: Int -> (String, Int)
reciprocal n | n > 1 = ('0' : '.' : digits, recur)
| otherwise = error
"attempting to compute reciprocal of number &lt;= 1"
(digits, recur) = divide n 1 []
divide :: Int -> Int -> [Int] -> (String, Int)
divide n c cs | c `elem` cs = ([], position c cs)
| r == 0 = (show q, 0)
| r /= 0 = (show q ++ digits, recur)
(q, r) = (c*10) `quotRem` n
(digits, recur) = divide n r (c:cs)
position :: Int -> [Int] -> Int
position n (x:xs) | n==x = 1
| otherwise = 1 + position n xs
showRecip :: Int -> String
showRecip n =
"1/" ++ show n ++ " = " ++
if r==0 then d else take p d ++ "(" ++ drop p d ++ ")"
p = length d - r
(d, r) = reciprocal n
main = do
number &lt;- readLn
putStrLn (showRecip number)
` <para>The HPC intrumentation is enabled using the -fhpc flag.
$ ghc -fhpc Recip.hs --make
<para>HPC index (.mix) files are placed placed in .hpc subdirectory. These can be considered like
the .hi files for HPC. They contain information about what parts of the haskell each modules.
$ ./Recip
= 0.(3)
<para>Now for a textual summary of coverage:</para>
$ hpc-report Recip
80% expressions used (81/101)
12% boolean coverage (1/8)
14% guards (1/7), 3 always True,
1 always False,
2 unevaluated
0% 'if' conditions (0/1), 1 always False
100% qualifiers (0/0)
55% alternatives used (5/9)
100% local declarations used (9/9)
100% top-level declarations used (5/5)
<para>Finally, we generate a marked-up version of the source.</para>
$ hpc-markup Recip
writing Recip.hs.html
<graphic fileref="images/Recip.png"></graphic>
<sect1 id="ticky-ticky">
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