Implementing HtDP Teachpacks, Libraries, and Customized Teaching Languages

DrRacket has two different mechanisms for making available additional functions and functionality to students using the teaching languages.

• HtDP Teachpacks are added to a student’s program by clicking on the “Language” menu and selecting “add Teachpack”. Students can then install a new Teachpack by clicking “Add Teachpack to List” and choosing the Teachpack file from the filesystem.

• HtDP Libraries are brought into the student’s program using a require statement.

Under the hood, HtDP Teachpacks and HtDP Libraries are implemented the same way, using normal Racket Modules.

This manual describes library support for authors of HtDP Teachpacks, libraries, and customized teaching languages. Use the HtDP error reporting functions to create error messages that integrate smoothly with those of the teaching languages. Before composing new error messages, we recommend you read the error message composition guidelines that informed the design of the error messages of DrRacket’s teaching languages.

1 Error Message Composition Guidelines

This section lists some guidelines for writing good error messages for novices, as informed by our research. Please follow these guidelines when you write code that is intended for beginners, including libraries and teachpacks. It ensures that error messages from your code fits messages from the student languages and from other teachpacks.

1.1 General Guidelines

• Frustrated students will peer at the error message for clues on how to proceed. Avoid offering hints, and avoid proposing any specific modification. Students will follow well-meaning-but-wrong advice uncritically, if only because they have no reason to doubt the authoritative voice of the tool.

• Be concise and clear. Students give up reading error messages if the text is too long, uses obscure words, or employs difficult grammar.

1.2 Message Structure and Form

• Start the message with the name of the construct whose constraint is being violated, followed by a colon.

• State the constraint that was violated (“expected a...”), then contrast with what was found. For example, “this function expects two arguments, but found only one”. If needed, explain how what was found fails to satisfy the constraint. Write somewhat anthropomorphically with an objective voice that is neither friendly nor antagonistic.

• If an expression contains multiple errors, report the leftmost error first. E.g., the error in (define 1 2 3) is “expected the variable name, but found a number”, not “expected 2 parts after define, but found 3”. Before raising an error about a sub-part of a macro, call syntax-local-expand-expression on sub-expressions to its left, so that such errors are shown first.

• State the number of parts instead of saying “found too many parts”. Write the code necessary to make plurals agree.

1.3 Words For Describing Code

Use only the following vocabulary words to describe code:

• Use binding for the square-braced pair in a let and similar binding forms.

• Use ‘argument’ for actual arguments and ‘variable’ for formal arguments in the header and body of a definition.

• Use ‘part’ when speaking about an s-expression that is not an expression, either because it is malformed, because it occurs in a non-expression position, or because it is a valid piece of syntax for a macro invocation. A well-formed and well-placed call to a function, primitive, or macro is not a ‘part’, it is an ‘expression’.

1.4 Words For Describing Runtime Behavior

Use the following vocabulary words to describe how code runs:

• When specifying a function’s behavior, say “the function takes ... and produces ...”

• When describing a contract violation, say “the function expects ... but received ...”

• As much as possible, identify expressions and the value they evaluate to, e.g. “the value of (f x) is 5”. If it is necessary to mention evaluation order, such as when the context discusses mutable state, say that the expression “evaluates to” a value. Function calls are a special case of expression. Prefer “the function call produces ...” to “the function call evaluates to ...”, except when trying to draw attention to the evaluation of the arguments.

• set! and set-structure-name-field-name! ‘mutate’ variables and structure instances, respectively. Avoid using the verb ‘sets’ when discussing mutation, and reserve the verbs ‘changes’ and ‘updates’ for functional updates.

1.5 Prohibited Words

These guidelines use few terms intentionally, emphasizing commonality among concepts rather than technical precision (which most students do not appreciate anyway).

1.6 General Vocabulary Guidelines

• Avoid modifiers that are not necessary to disambiguate. Write ‘variable’ instead of ‘local variable’, ‘defined variable’, or ‘input variable’. Write ‘clause’ instead of ‘question-answer clause’. If they appear necessary for disambiguation, try to find some other way to achieve this (and drop the modifier).

• When introducing macros with sub-parts, reuse existing vocabulary words, such as ‘clause’ or ‘binding’ (if appropriate), or just ‘part’, instead of defining new terms.

• Use ‘name’ only when describing the syntax of a definition form. For example, the define form in BSL should say “expected at least one variable after the function name”. Outside of the definition form, simply use the word ‘function’ rather than distinguish between (1) a function, (2) the variable that binds the function, and (3) the name of that variable.

  [Rationale: Students learn this distinction when they learn about lambda. The first is the lambda implicit in the definition, the second is the variable introduced by the definition that can appear as the first argument to set!, the third is the particular sequence of letters. But BSL should avoid this complexity, and ASL’s error messages should maintain consistency with BSL.]

• Avoid introducing technical vocabulary, even if well-known to a mathematician.

1.7 Punctuation

• Do not use any punctuation beyond those of the normal English language. Do not write <> around type names, and do not write quotes around keywords.

1.8 Supporting Research

2 Error Reporting Functions

To provide uniform error messages from teachpacks, this module provides several functions:

MF: These library and its uses needs to be cleaned up.

3 Testing

The library re-exports the identifiers from test-engine/racket-tests.

In addition, it exports:

4 HtDP Languages as Libraries

4.1 HtDP Beginning Student

The lang/htdp-beginner module provides the Beginning Student Language; see Beginning Student.

4.2 HtDP Beginning Student with Abbreviations

The lang/htdp-beginner-abbr module provides the Beginning Student with Abbreviations language; see Beginning Student with List Abbreviations.

4.3 HtDP Intermediate Student

The lang/htdp-intermediate module provides the Intermediate Student language; see Intermediate Student.

4.4 HtDP Intermediate Student with Lambda

The lang/htdp-intermediate-lambda module provides the Intermediate Student with Lambda language; see Intermediate Student with Lambda.

4.5 HtDP Advanced Student

The lang/htdp-advanced module provides the Advanced Student language; see Advanced Student.

4.6 Pretty Big Text (Legacy Language)

The lang/plt-pretty-big-text module is similar to the HtDP Advanced Student language, but with more of Racket’s libraries in legacy form. It provides the bindings of mzscheme, mzlib/etc, mzlib/file, mzlib/list, mzlib/class, mzlib/unit, mzlib/include, mzlib/defmacro, mzlib/pretty, mzlib/string, mzlib/thread, mzlib/math, mzlib/match, mzlib/shared, and lang/posn.

4.7 Pretty Big (Legacy Language)

The lang/plt-pretty-big module extends lang/plt-pretty-big-text with racket/gui/base and lang/imageeq. This language corresponds to the Pretty Big legacy language in DrRacket.

4.8 posns in HtDP Languages

4.9 Image Equality in HtDP Languages

4.10 Primitives in HtDP Beginner

The lang/prim module several syntactic forms for use by the implementors of teachpacks, when the teachpack is to be used with the Beginner Student Language. In Beginner Student, primitive names (for built-in procedures) are distinguished from other types of expressions, so that they can be syntactically restricted to application positions.

For each non-procedure argument, the corresponding arg should be an underscore. For each procedure argument, the corresponding arg should be the usual name of the procedure.

This form is mainly useful for implementing syntactic forms that, like the application of a higher-order primitive, allow first-order bindings to be used in an expression position.

5 Color and Alpha Color Structs