Code coverage tools allow a programmer 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 Haskell Program Coverage (HPC) toolkit, which is included with GHC. HPC tools can be used to render the generated code coverage information into human understandable format.
Correctly instrumented code 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 primary 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.
For an example we have a program, called Recip.hs, which computes exact decimal representations of reciprocals, with recurring parts indicated in brackets.
reciprocal :: Int -> (String, Int)
reciprocal n | n > 1 = ('0' : '.' : digits, recur)
| otherwise = error
"attempting to compute reciprocal of number <= 1"
where
(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)
where
(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 ++ ")"
where
p = length d - r
(d, r) = reciprocal n
main = do
number <- readLn
putStrLn (showRecip number)
main
The HPC instrumentation is enabled using the -fhpc flag.
$ ghc -fhpc Recip.hs --make
HPC index (.mix) files are placed placed in .hpc subdirectory. These can be considered like the .hi files for HPC.
$ ./Recip 1/3 = 0.(3)
We can generate a textual summary of coverage:
$ 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)
We can also generate a marked-up version of the source.
$ hpc markup Recip writing Recip.hs.html
This generates one file per Haskell module, and 4 index files, hpc_index.html, hpc_index_alt.html, hpc_index_exp.html, hpc_index_fun.html.
Turning on code coverage is easy, use the -fhpc flag. Instrumented and non-instrumented can be freely mixed. When compiling the Main module GHC automatically detects when there is an hpc compiled file, and adds the correct initialization code.
The hpc toolkit uses a cvs/svn/darcs-like interface, where a single binary contains many function units.
$ hpc Usage: hpc COMMAND ... Commands: help Display help for hpc or a single command Reporting Coverage: report Output textual report about program coverage markup Markup Haskell source with program coverage Processing Coverage files: sum Sum multiple .tix files in a single .tix file combine Combine two .tix files in a single .tix file map Map a function over a single .tix file Coverage Overlays: overlay Generate a .tix file from an overlay file draft Generate draft overlay that provides 100% coverage Others: show Show .tix file in readable, verbose format version Display version for hpc
In general, these options act on .tix file after an instrumented binary has generated it, which hpc acting as a conduit between the raw .tix file, and the more detailed reports produced.
The hpc tool assumes you are in the top-level directory of the location where you built your application, and the .tix file is in the same top-level directory. You can use the flag --srcdir to use hpc for any other directory, and use --srcdir multiple times to analyse programs compiled from difference locations, as is typical for packages.
We now explain in more details the major modes of hpc.
hpc report gives a textual report of coverage. By default, all modules and packages are considered in generating report, unless include or exclude are used. The report is a summary unless the --per-module flag is used. The --xml-output option allows for tools to use hpc to glean coverage.
$ hpc help report
Usage: hpc report [OPTION] .. <TIX_FILE> [<MODULE> [<MODULE> ..]]
Options:
--per-module show module level detail
--decl-list show unused decls
--exclude=[PACKAGE:][MODULE] exclude MODULE and/or PACKAGE
--include=[PACKAGE:][MODULE] include MODULE and/or PACKAGE
--srcdir=DIR path to source directory of .hs files
multi-use of srcdir possible
--hpcdir=DIR sub-directory that contains .mix files
default .hpc [rarely used]
--xml-output show output in XML
hpc markup marks up source files into colored html.
$ hpc help markup
Usage: hpc markup [OPTION] .. <TIX_FILE> [<MODULE> [<MODULE> ..]]
Options:
--exclude=[PACKAGE:][MODULE] exclude MODULE and/or PACKAGE
--include=[PACKAGE:][MODULE] include MODULE and/or PACKAGE
--srcdir=DIR path to source directory of .hs files
multi-use of srcdir possible
--hpcdir=DIR sub-directory that contains .mix files
default .hpc [rarely used]
--fun-entry-count show top-level function entry counts
--highlight-covered highlight covered code, rather that code gaps
--destdir=DIR path to write output to
hpc sum adds together any number of .tix files into a single .tix file. hpc sum does not change the original .tix file; it generates a new .tix file.
$ hpc help sum
Usage: hpc sum [OPTION] .. <TIX_FILE> [<TIX_FILE> [<TIX_FILE> ..]]
Sum multiple .tix files in a single .tix file
Options:
--exclude=[PACKAGE:][MODULE] exclude MODULE and/or PACKAGE
--include=[PACKAGE:][MODULE] include MODULE and/or PACKAGE
--output=FILE output FILE
--union use the union of the module namespace (default is intersection)
hpc combine is the swiss army knife of hpc. It can be used to take the difference between .tix files, to subtract one .tix file from another, or to add two .tix files. hpc combine does not change the original .tix file; it generates a new .tix file.
$ hpc help combine
Usage: hpc combine [OPTION] .. <TIX_FILE> <TIX_FILE>
Combine two .tix files in a single .tix file
Options:
--exclude=[PACKAGE:][MODULE] exclude MODULE and/or PACKAGE
--include=[PACKAGE:][MODULE] include MODULE and/or PACKAGE
--output=FILE output FILE
--function=FUNCTION combine .tix files with join function, default = ADD
FUNCTION = ADD | DIFF | SUB
--union use the union of the module namespace (default is intersection)
hpc map inverts or zeros a .tix file. hpc map does not change the original .tix file; it generates a new .tix file.
$ hpc help map
Usage: hpc map [OPTION] .. <TIX_FILE>
Map a function over a single .tix file
Options:
--exclude=[PACKAGE:][MODULE] exclude MODULE and/or PACKAGE
--include=[PACKAGE:][MODULE] include MODULE and/or PACKAGE
--output=FILE output FILE
--function=FUNCTION apply function to .tix files, default = ID
FUNCTION = ID | INV | ZERO
--union use the union of the module namespace (default is intersection)
Overlays are an experimental feature of HPC, a textual description of coverage. hpc draft is used to generate a draft overlay from a .tix file, and hpc overlay generates a .tix files from an overlay.
% hpc help overlay
Usage: hpc overlay [OPTION] .. <OVERLAY_FILE> [<OVERLAY_FILE> [...]]
Options:
--srcdir=DIR path to source directory of .hs files
multi-use of srcdir possible
--hpcdir=DIR sub-directory that contains .mix files
default .hpc [rarely used]
--output=FILE output FILE
% hpc help draft
Usage: hpc draft [OPTION] .. <TIX_FILE>
Options:
--exclude=[PACKAGE:][MODULE] exclude MODULE and/or PACKAGE
--include=[PACKAGE:][MODULE] include MODULE and/or PACKAGE
--srcdir=DIR path to source directory of .hs files
multi-use of srcdir possible
--hpcdir=DIR sub-directory that contains .mix files
default .hpc [rarely used]
--output=FILE output FILE
HPC does not attempt to lock the .tix file, so multiple concurrently running binaries in the same directory will exhibit a race condition. There is no way to change the name of the .tix file generated, apart from renaming the binary. HPC does not work with GHCi.