7 November 2018 (after class)
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Few computer science graduates will design and implement a general-purpose programming language during their careers. However, many graduates will design and implement—and all likely will use—special-purpose languages in their work.
These special-purpose languages are often called domain-specific languages (or DSLs). For more discussion of DSL concepts and terminology, see the accompanying notes on Domain-Specific Languages.
In this project, we design and implement a simple internal DSL. This DSL describes simple “programs” using a set of Haskell algebraic data types. We express a program as an abstract syntax tree (AST) using the DSL’s data types.
The package first builds a set of functions for creating and manipulating the abstract syntax trees for the exams. It then extends the package to translate the abstract syntax trees to HTML.
Note: A similar project is the SandwichDSL case study.
Suppose Professor Harold Pedantic decides to create a DSL to encode his (allegedly vicious) multiple choice examinations. Since his course uses Haskell to teach programming language organization, he wishes to implement the language processor in Haskell. Professor Pedantic is too busy to do the task himself. He is also cheap, so he assigns us, the students in his class, the task of developing a prototype.
In the initial prototype, we do not concern ourselves with the concrete syntax of the Exam DSl. We focus on design of the AST as a Haskell algebraic data type. We seek to design a few useful functions to manipulate the AST and output an exam as HTML.
First, let’s focus on multiple-choice questions. For this prototype, we can assume a question has the following components:
the text of the question
a group of several choices for the answer to the question, exactly one of which should be be a correct answer to the question
a group of tags identifying topics covered by the question
We can state a question using the Haskell data type Question, which has a single constructor Ask. It has three components—a list of applicable topic tags, the text of the question, and a list of possible answers to the question.
We use the type QText to describe the text of a question. We also use the type Tag to describe the topic tags we can associate with a question.
We can then state a possible answer to the question using the data type Choice, which has a single constructor Answer. It has two components—the text of the answer and a Boolean value that indicates whether this is a correct answer to the question (i.e. True) or not.
As above, we use the type AText to describe the text of an answer.
For example, we could encode the question “Which of the following is a required course?” as follows.
Ask ["curriculum"]
"Which of the following is a required course?"
[ Answer "CSci 323" False,
Answer "CSci 450" True,
Answer "CSci 525" False ]The example has a single topic tag "curriculum" and three possible answers, the second of which is correct.
We can develop various useful functions on these data types. Most of these are left as exercises.
For example, we can define a function correctChoice that takes a Choice and determines whether it is marked as a correct answer or not.
We can also define function lenQuestion that takes a question and returns the number of possible answers are given. This function has the following signature.
We can then define a function to check whether a question is valid. That is, the question must have:
a non-nil text
at least 2 and no more than 10 possible answers
exactly one correct answer
It has the type signature.
We can also define a function to determine whether or not a question has a particular topic tag.
To work with our lists of answers (and other lists in our program), let’s define function eqBag with the following signature.
This is a “bag equality” function for two polymorphic lists. That is, the lists are collections of elements that can be compared for equality and inequality, but not necessarily using ordered comparisons. There may be elements repeated in the list.
Now, what does it mean for two questions to be equal?
For our prototype, we require that the two questions have the same question text, the same collection of tags, and the same collection of possible answers with the same answer marked correct. However, we do not require that the tags or possible answers appear in the same order.
We note that type Choice has a derived instance of class Eq. Thus we can give an instance definition to make Question an instance of class Eq.
Now, let’s consider the examination as a whole. It consists of a title and a list of questions. We thus define the data type Exam as follows.
We can encode an exam with two questions as follows.
Quiz "Curriculum Test" [
Ask ["curriculum"]
"Which one of the following is a required course?"
[ Answer "CSci 323" False,
Answer "CSci 450" True,
Answer "CSci 525" False ],
Ask ["language","course"]
"What one of the following languages is used in CSci 450?"
[ Answer "Lua" False,
Answer "Elm" False,
Answer "Haskell" True ]
]We can define function selectByTags selects questions from an exam based on the occurrence of the specified topic tags.
The function application selectByTags tags exam takes a list of zero or more tags and an exam and returns an exam with only those questions in which at least one of the given tags occur in a Question’s tag list.
We can define function validExam that takes an exam and determines whether or not it is valid. It is valid if and only if all questions are valid. The function has the following signature.
To assist in grading an exam, we can also define a function makeKey that takes an exam and creates a list of (number,letter) pairs for all its questions. In a pair, number is the problem number, a value that starts with 1 and increases for each problem in order. Similarly, letter is the answer identifier, an uppercase alphabetic character that starts with A and increases for each choice in order. The function returns the tuples arranged by increasing problem number.
The function has the following signature.
For the example exam above, makeKey should return [(1,'B'),(2,'C')].
Define the following functions in a module named ExamDSL (in a file named ExamDSL.hs).
Develop function correctChoice :: Choice -> Bool as defined above.
Develop function lenQuestion :: Question -> Int as defined above.
Develop function validQuestion :: Question -> Bool as defined above.
Develop function hasTag :: Question -> Tag -> Bool as defined above.
Develop function eqBag :: Eq a => [a] -> [a] -> Bool as defined above.
Give an instance declaration to make data type Question an instance of class Eq.
Develop function selectByTags :: [Tag] -> Exam -> Exam as defined above.
Develop function validExam :: Exam -> Bool as defined above.
Develop function makeKey :: Exam -> [(Int,Char)] as defined above.
Professor Pedantic wants to take an examination expressed with the Exam DSL, as described above, and output it as HTML.
Again, consider the following Exam value.
Quiz "Curriculum Test" [
Ask ["curriculum"]
"Which one of the following courses is required?"
[ Answer "CSci 323" False,
Answer "CSci 450" True,
Answer "CSci 525" False ],
Ask ["language","course"]
"What one of the following is used in CSci 450?"
[ Answer "Lua" False,
Answer "Elm" False,
Answer "Haskell" True ]
]We want to convert the above to the following HTML.
<html lang="en">
<body>
<h1>Curriculum Test</h1>
<ol type="1">
<li>Which one of the following courses is required?
<ol type="A">
<li>CSci 323</li>
<li>CSci 450</li>
<li>CSci 525</li>
</ol>
</li>
<li>What one of the following is used in CSci 450?
<ol type="A">
<li>Lua</li>
<li>Elm</li>
<li>Haskell</li>
</ol>
</li>
</ol>
</body>
</html>This would render in a browser something like the following.
Curriculum Test
Professor Pedantic developed a module of HTML template functions named SimpleHTML to assist us in this process. (See file SimpleHTML.hs.)
A function application to_html lang content wraps the content (HTML in a string) inside a pair of HTML tags <html> and </html> with lang attribute set to langtype, defaulting to English (i.e. "en"). This function and the data types are defined in the following.
type HTML = String
data LangType = English | Spanish | Portuguese | French
deriving (Eq, Show)
langmap = [ (English,"en"), (Spanish,"es"), (Portuguese,"pt"),
(French,"fr") ]
to_html :: LangType -> HTML -> HTML
to_html langtype content =
"<html lang=\"" ++ lang ++ "\">" ++ content ++ "</html>"
where lang = case lookup langtype langmap of
Just l -> l
Nothing -> "en"For the above example, the to_html function generates the the outer layer:
Function application to_body content wraps the content inside a pair of HTML tags <body> and </body>.
Function application to_heading level title wraps string title inside a pair of HTML tags <hN> and </hN> where N is in the range 1 to 6. If level is outside this range, it defaults to the nearest valid value.
to_heading:: Int -> String -> HTML
to_heading level title = open ++ title ++ close
where lev = show (min (max level 1) 6)
open = "<h" ++ lev ++ ">"
close = "</h" ++ lev ++ ">"Function application to_list listtype content wraps the content inside a pair of HTML tags <ul> and </ul> or <ol> and </ol>. For <ol> tags, it sets the type attribute based on the value of the listtype argument.
data ListType = Decimal | UpRoman | LowRoman
| UpLettered | LowLettered | Bulleted
deriving (Eq, Show)
to_list :: ListType -> HTML -> HTML
to_list listtype content = open ++ content ++ close
where
(open,close) =
case listtype of
Decimal -> ("<ol type=\"1\">", "</ol>")
UpRoman -> ("<ol type=\"I\">", "</ol>")
LowRoman -> ("<ol type=\"i\">", "</ol>")
UpLettered -> ("<ol type=\"A\">", "</ol>")
LowLettered -> ("<ol type=\"a\">", "</ol>")
Bulleted -> ("<ul>", "</ul>")Finally, function application to_li content wraps the content inside a pair of HTML tags <li> and </li>.
By importing the SimpleHTML module, we can now develop functions to output an Exam as HTML.
If we start at the leaves of the Exam AST (i.e. from the Choice data type), we can define a function choice2html function as follows in terms of to_li.
Using choice2html and the SimpleHTML module, we can define question2html with the following signature.
Then using question2html and the SimpleHTML module, we can define question2html with the following signature.
Note: These two functions should add newline characters to the HTML output so that they look like the examples at the beginning of the “Outputting” section. Similarly, it should not output extra spaces. This both makes the string output more readable and makes it possible to grade the assignment using automated testing.
For example, the output of question2html for the first Question in the example above should appear as the following when printed with the putStr input-output command.
<li>Which one of the following courses is required?
<ol type="A">
<li>CSci 323</li>
<li>CSci 450</li>
<li>CSci 525</li>
</ol>In addition, you may want to output the result of exam2html to a file to see how it displays in a browser a particular exam.
Add the following functions to the module ExamDSL developed in Exercise Set A.
Develop function question2html :: Question -> HTML as defined above.
Develop function exam2html :: Exam -> HTML as defined above.
ExamDSL_base.hs is the skeleton to flesh out for a solution to this project.
SimpleHTML.hs is the module of HTML string templates.
I developed this project description in Fall 2018 motivated by the Sandwich DSL project and a set of questions I gave on an exam in the past.
I maintain this document as text in Pandoc’s dialect of Markdown using embedded LaTeX markup for the mathematical formulas and then translate the notes to HTML, PDF, and other forms as needed. The HTML version of this document may require use of a browser that supports the display of MathML.
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