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Falderal Literate Test Format

This document describes the proposed Falderal Literate Test Format.


This document is a draft. It is nominally "version 0.8" because it describes something that version 0.8 of Test.Falderal mostly implements. We will deign to note which sections of this document the current released version of Test.Falderal implements, and which it does not. However, this document is a work in progress, subject to change, and subject to get out of sync with Test.Falderal. It should not be considered to be anything except a draft until it is described as "version 1.0".


A Falderal Literate Test Suite is a text file where some of the lines have special meaning to the Falderal Literate Test Format. Certain groupings of the lines defined by this format are intended to denote tests. A tool which claims to understand this format may chose to extract these lines, interpret them as tests, and execute these tests and report which and how many of those tests passed or failed. It may also reformat these lines to produce an output file which is no longer in the Falderal Literate Test Format, but in some other format, generally to present the tests and their descriptions in a nicely human-readable fashion.


Lines which have special meaning to the Falderal Literate Test Format always begin with an indent of four (4) spaces from the leftmost column followed immediately by a series of distinguished characters, called an introducer. The introducers which have meaning to the Falderal Literate Test Format are as follows:

  • -> (hyphen, greater-than sign): pragma
  • | (vertical bar, space): input text
  • = (equals sign, space): expected output text
  • ? (question mark, space): expected error text
  • > (greater-than sign, space): Bird-style embedded code

If the same introducer occurs on multiple adjacent lines, the text after each introducer is concatenated to form one chunk of relevant text. This allows, for example, multi-line text to be given as input or expected output of a test.

There are some restrictions on the order in which introducers can sensibly occur. Expected output or error text must follow input text, with no intervening introducers or non-introducer lines. See the sections for these introducers, below, for more details.

Bird-style embedded code is not considered part of a test by the Falderal Literate Test Format, but may be recognized as such by a formatting tool, for purposes of formatting.

Lines which do not begin with an introducer are classified as either blank or non-blank. A line is blank if it contains no characters, or if it contains only whitespace. A group of non-blank lines is referred to as a paragraph.



The encoding pragma allows a Falderal file to specify what encoding is used for the characters in it. An implementation of Falderal is not expected to be able to handle any coding other than UTF-8, however, this pragma is included for the benefit of text editors and other tools, to indicate that the document is in fact in UTF-8 encoding.


-> encoding: UTF-8


The Functionality-definition pragma allows a Falderal file to describe ways in which a functionality being tested is implemented. It has the following syntax:

-> Functionality /functionality-name/ is implemented by /functionality-type/ /functionality-specifier/

functionality-type may be either Haskell function or shell command. The format of the functionality-specifier differs with each of these. The functionality-name is arbitrary text enclosed within double quotes, which may be referenced in a later Tests-for pragma.

Note that the Functionality-definitions given in a Falderal file should not be considered exhaustive, or even requisite, by a tool. The tool may accept additional definitions of a functionality, referencing it by its name, from an external source such as the command line or a configuration file, and may be instructed to ignore certain Functionality-definitions in a Falderal file (if, for example, certain implementation are not currently available or of interest to the user.) Indeed, the functionality referred to by a functionality-name in a Tests-for pragma need not be defined by any Functionality-definition pragma in the same Falderal file, and this situation requires the functionality to be specified to the tool in some other manner.

Haskell functions

For Haskell functions, the functionality-specifier is in the format Module:functionName. The function should have a signature of String -> String. If the function raises an exception, the text of that exception will be compared against expected error output, if any. For example:

-> Functionality 'Reversing a string' is implemented by Haskell function Data.Backwards:reverseString

Shell commands

For shell commands, the functionality-specifier is in the format "command arg1 arg2 ... argn". Any line of legal Bourne shell syntax may be used, so pipes, redirection, etc., are supported. Note that the double quotation mark characters used to enclosed the command have meaning only to the Falderal format — they are not part of the command, are not passed to the shell, and do not require double quotation mark characters that are enclosed by them to be escaped with a backslash.

Certain subsequences, called variables, if present in the command string, will be expanded before execution, and will alter how the command reads the text of the test and produces its output, to be compared with the expected output.


The variable %(test-file) will be replaced by the name of a file which contains the text of the test. This may be a temporary file created solely for this purpose by the Falderal implementation.


The variable %(test-text) will be replaced by the actual text of the test. It is assumed that %(test-text) will appear inside single quotes in the command string, so any single quotes in the text of the test will be escaped by the Falderal implementation by preceding them with backslashes.

If neither of the variables %(test-file) nor %(test-text) appear in the command string, the test text will be provided on the standard input of the shell command.


The variable %(output-file) will be replaced by the name of a file (temporary file) to which the test results will be written. If it does not appear in the command string, the output text will be read from the standard output of the command.

How shell commands support error output is not yet standardized.

For example:

-> Functionality 'Prepending foo.txt' is implemented by shell command "cat foo.txt %(test-file) > %(output-file)"


The Tests-for pragma determines what functionality will be used to run all following tests, until the next Tests-for pragma. It has the following syntax:

-> Tests for _functionality-name_

The functionality-name refers to a functionality, which may be specified by a Functionality-definition pragma elsewhere in the Falderal file.

For example:

-> Tests for 'Reversing a string'

Alternatively, the following direct way of associating tests with an implementation of a functionality, may be used. However, this direct way of specifying a functionality is discouraged when there may be conceivably be multiple implementation of the functionality.

-> Tests for _functionality-type_ _functionality-specifier_

functionality-type and functionality-specifier have the same meaning as given in the description of the Functionality-definition pragma.

For example:

-> Tests for Haskell function Data.Backwards:reverseString

Input and Expected Text

Each section of input text should be followed immediately by either a section of expected output or expected error output. It may also be preceded by a paragraph of text; the intent of the Falderal Literate Test Format is that this text should describe the test, or rather, the aspect of the behaviour of the system that the test is meant to check. It is therefore reasonably that this text should be displayed along with the contents (input text and expected text) of the test, in, for example, a test result report.


(This section is non-normative.)

Typically, a file in Falderal Literate Test Format will also be in Markdown format.

The format of the lines which comprise the Falderal Literate Test Format was chosen to not conflict with many other common text formats (including but not limited to Bird-style Literate Haskell, and Markdown); thus literate test suites may be embedded in a wide variety of other formats. However, there are inevitably some conflicts with some textual formats; for example, when embedded in C code and many other languages, Falderal entries should be preceded by /* and followed by */, to ensure that they are regarded as comments. Also, reStructuredText uses the | line prefix to denote preformatted plain text.

The format of pragmas was chosen such that they could be read literately, and as such, a formatting tool could format them in the output document with little if any change.

The Haskell function functionality type is essentially an optimization, and a convenience to avoid writing Haskell code for a system program which reads strings, passes them to a function, and outputs the result. However, restricting oneself to Haskell functions provides a fairly good guarantee that the tests are idempotent; shell commands may change the state of the system. (It is of course recommended that when shell commands are used, care is taken to leave the system in the same state as it was when testing started.)

Here is why directly specifying the functionality implementation in a Tests-for pragma is discouraged. Saying -> Tests for Haskell function Foo:blah ties a set of tests to a particular function which is being tested, but a large part of the point of Falderal is to let you write tests for a language, and that language might have many implementations. Decoupling them allows you to change what actual functions or programs are being tested, and basically allow you to have multiple implementations of a language and use the same tests for all of them.