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Doctest: Test interactive Haskell examples

doctest is a tool that checks examples and properties in Haddock comments. It is similar in spirit to the popular Python module with the same name.

Getting started

Installation

doctest is available from Hackage. Install it with:

cabal update && cabal install --ignore-project doctest

Make sure that Cabal's installdir is on your PATH.

On Linux / macOS / BSD:

 # requires cabal-install version 3.12, or later
export PATH="$(cabal -v0 path --installdir):$PATH"

or

export PATH="$HOME/.local/bin:$PATH"

On Windows with PowerShell:

# requires cabal-install version 3.12, or later
$Env:PATH = "$(cabal -v0 path --installdir)" + ";" + $Env:PATH

A basic example

Below is a small Haskell module. The module contains a Haddock comment with some examples of interaction. The examples demonstrate how the module is supposed to be used.

-- src/Fib.hs
module Fib where

-- | Compute Fibonacci numbers
--
-- Examples:
--
-- >>> fib 10
-- 55
--
-- >>> fib 5
-- 5
fib :: Int -> Int
fib 0 = 0
fib 1 = 1
fib n = fib (n - 1) + fib (n - 2)

(A comment line starting with >>> denotes an expression. All comment lines following an expression denote the result of that expression. Result is defined by what a REPL (e.g. ghci) prints to stdout and stderr when evaluating that expression.)

With doctest you can check whether the implementation satisfies the given examples:

doctest src/Fib.hs

Running doctest for a Cabal package

The easiest way to run doctest for a Cabal package is via cabal repl --with-compiler=doctest.

This doesn't make a big difference for a simple package, but in more involved situations cabal will make sure that all dependencies are available and it will pass any required GHC options to doctest.

A simple .cabal file for Fib looks like this:

-- fib.cabal
cabal-version: 1.12

name:           fib
version:        0.0.0
build-type:     Simple

library
  build-depends: base == 4.*
  hs-source-dirs: src
  exposed-modules: Fib
  default-language: Haskell2010

With a .cabal file in place, it is possible to run doctest via cabal repl:

$ cabal repl --with-compiler=doctest
...
Examples: 2  Tried: 2  Errors: 0  Failures: 0

Notes:

So a more robust way to call doctest is as follows:

cabal install doctest --ignore-project --overwrite-policy=always && cabal build && cabal repl --build-depends=QuickCheck --build-depends=template-haskell --with-compiler=doctest --repl-options='-w -Wdefault'

(This is what you want to use on CI.)

Passing doctest options to cabal repl

You can pass doctest options like --fast, --preserve-it and --verbose to cabal repl via --repl-options.

Example:

$ cabal repl --with-compiler=doctest --repl-options=--verbose
### Started execution at src/Fib.hs:7.
### example:
fib 10
### Successful!

### Started execution at src/Fib.hs:10.
### example:
fib 5
### Successful!

# Final summary:
Examples: 2  Tried: 2  Errors: 0  Failures: 0

Cabal integration

NOTE: This feature is experimental.

NOTE: This feature requires cabal-install version 3.12 or later.

$ cabal install --ignore-project doctest --flag cabal-doctest

$ cabal doctest
Examples: 2  Tried: 2  Errors: 0  Failures: 0

$ cabal doctest -w ghc-8.6.5
Examples: 2  Tried: 2  Errors: 0  Failures: 0

$ cabal doctest --repl-options=--verbose
### Started execution at src/Fib.hs:7.
### example:
fib 10
### Successful!

### Started execution at src/Fib.hs:10.
### example:
fib 5
### Successful!

# Final summary:
Examples: 2  Tried: 2  Errors: 0  Failures: 0

$ cabal doctest --build-depends transformers
Examples: 2  Tried: 2  Errors: 0  Failures: 0

Writing examples and properties

Example groups

Examples from a single Haddock comment are grouped together and share the same scope. E.g. the following works:

-- |
-- >>> let x = 23
-- >>> x + 42
-- 65

If an example fails, subsequent examples from the same group are skipped. E.g. for

-- |
-- >>> let x = 23
-- >>> let n = x + y
-- >>> print n

print n is skipped, because let n = x + y fails (as y is not in scope).

A note on performance

By default, doctest calls :reload between each group to clear GHCi's scope of any local definitions. This ensures that previous examples cannot influence later ones. However, it can lead to performance penalties if you are using doctest in a project with many modules. One possible remedy is to pass the --fast flag to doctest, which disables calling :reload between groups. If doctests are running too slowly, you might consider using --fast. (With the caveat that the order in which groups appear now matters!)

However, note that due to a bug on GHC 8.2.1 or later, the performance of --fast suffers significantly when combined with the --preserve-it flag (which keeps the value of GHCi's it value between examples).

Setup code

You can put setup code in a named chunk with the name $setup. The setup code is run before each example group. If the setup code produces any errors/failures, all tests from that module are skipped.

Here is an example:

module Foo where

import Bar.Baz

-- $setup
-- >>> let x = 23 :: Int

-- |
-- >>> foo + x
-- 65
foo :: Int
foo = 42

Note that you should not place setup code inbetween the module header (module ... where) and import declarations. GHC will not be able to parse it (issue #167). It is best to place setup code right after import declarations, but due to its declarative nature you can place it anywhere inbetween top level declarations as well.

Multi-line input

GHCi supports commands which span multiple lines, and the same syntax works for doctest:

-- |
-- >>> :{
--  let
--    x = 1
--    y = 2
--  in x + y + multiline
-- :}
-- 6
multiline = 3

Note that >>> can be left off for the lines following the first: this is so that haddock does not strip leading whitespace. The expected output has whitespace stripped relative to the :}.

Some peculiarities on the ghci side mean that whitespace at the very start is lost. This breaks the example broken, since the x and y aren't aligned from ghci's perspective. A workaround is to avoid leading space, or add a newline such that the indentation does not matter:

{- | >>> :{
let x = 1
    y = 2
  in x + y + works
:}
6
-}
works = 3

{- | >>> :{
 let x = 1
     y = 2
  in x + y + broken
:}
3
-}
broken = 3

Multi-line output

If there are no blank lines in the output, multiple lines are handled automatically.

-- | >>> putStr "Hello\nWorld!"
-- Hello
-- World!

If however the output contains blank lines, they must be noted explicitly with <BLANKLINE>. For example,

import Data.List ( intercalate )

-- | Double-space a paragraph.
--
--   Examples:
--
--   >>> let s1 = "\"Every one of whom?\""
--   >>> let s2 = "\"Every one of whom do you think?\""
--   >>> let s3 = "\"I haven't any idea.\""
--   >>> let paragraph = unlines [s1,s2,s3]
--   >>> putStrLn $ doubleSpace paragraph
--   "Every one of whom?"
--   <BLANKLINE>
--   "Every one of whom do you think?"
--   <BLANKLINE>
--   "I haven't any idea."
--
doubleSpace :: String -> String
doubleSpace = (intercalate "\n\n") . lines

Matching arbitrary output

Any lines containing only three dots (...) will match one or more lines with arbitrary content. For instance,

-- |
-- >>> putStrLn "foo\nbar\nbaz"
-- foo
-- ...
-- baz

If a line contains three dots and additional content, the three dots will match anything within that line:

-- |
-- >>> putStrLn "foo bar baz"
-- foo ... baz

QuickCheck properties

Haddock has markup support for properties. Doctest can verify properties with QuickCheck. A simple property looks like this:

-- |
-- prop> \xs -> sort xs == (sort . sort) (xs :: [Int])

The lambda abstraction is optional and can be omitted:

-- |
-- prop> sort xs == (sort . sort) (xs :: [Int])

A complete example that uses setup code is below:

module Fib where

-- $setup
-- >>> import Control.Applicative
-- >>> import Test.QuickCheck
-- >>> newtype Small = Small Int deriving Show
-- >>> instance Arbitrary Small where arbitrary = Small . (`mod` 10) <$> arbitrary

-- | Compute Fibonacci numbers
--
-- The following property holds:
--
-- prop> \(Small n) -> fib n == fib (n + 2) - fib (n + 1)
fib :: Int -> Int
fib 0 = 0
fib 1 = 1
fib n = fib (n - 1) + fib (n - 2)

If you see an error like the following, ensure that QuickCheck is visible to doctest (e.g. by passing --build-depends=QuickCheck to cabal repl).

<interactive>:39:3:
    Not in scope: ‘polyQuickCheck’
    In the splice: $(polyQuickCheck (mkName "doctest_prop"))

<interactive>:39:3:
    GHC stage restriction:
      ‘polyQuickCheck’ is used in a top-level splice or annotation,
      and must be imported, not defined locally
    In the expression: polyQuickCheck (mkName "doctest_prop")
    In the splice: $(polyQuickCheck (mkName "doctest_prop"))

Hiding examples from Haddock

You can put examples into named chunks, and not refer to them in the export list. That way they will not be part of the generated Haddock documentation, but Doctest will still find them.

-- $
-- >>> 1 + 1
-- 2

Using GHC extensions

There's two sets of GHC extensions involved when running Doctest:

  1. The set of GHC extensions that are active when compiling the module code (excluding the doctest examples). The easiest way to specify these extensions is through LANGUAGE pragmas in your source files.

  2. The set of GHC extensions that are active when executing the Doctest examples. (These are not influenced by the LANGUAGE pragmas in the file.) The recommended way to enable extensions for Doctest examples is to switch them on like this:

-- |
-- >>> :seti -XTupleSections
-- >>> fst' $ (1,) 2
-- 1
fst' :: (a, b) -> a
fst' = fst

Alternatively you can pass any GHC options to Doctest, e.g.:

doctest -XCPP Foo.hs

These options will affect both the loading of the module and the execution of the Doctest examples.

If you want to omit the information which language extensions are enabled from the Doctest examples you can use the method described in Hiding examples from Haddock, e.g.:

-- $
-- >>> :seti -XTupleSections

Limitations

Doctest in the wild

You can find real world examples of Doctest being used below:

Development

Discuss your ideas first, ideally by opening an issue on GitHub.

Add tests for new features, and make sure that the test suite passes with your changes.

cabal build && cabal exec $(cabal list-bin spec)

Contributors

For up-to-date list, query

git shortlog -s