The global idea of Hedy is that we transpile Hedy code to Python code by adding syntactic elements when they are missing. In the code base, this works in steps:
- Hedy code is parsed based on the relevant grammar, creating an AST. This is done using the open source Lark parser
- Validity of the code is checked with the IsValid() function
- If the code is valid, it is transformed into Python with the relevant function.
This logic all resides in hedy.py.
The overarching goal of Hedy is to successively add syntactic complexity to a Python-like language, until novices have mastered Python itself. To reach that goal, Hedy follows these design principles:
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Concepts are offered at least three times in different forms:
Research from writing education has shown that it is best to offer concept in different forms over a long period of time. Furthermore it has been shown that a word needs to be read 7 times before it is stored in long-term memory .
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The initial offering of a concept is the simplest form possible:
Previous research has shown that syntax can be confusing for novices. Early levels thus are as syntax-free as possible to lower cognitive load.
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Only one aspect of a concept changes at a time:
In his paper on the spiral approach Shneiderman argued for small steps in teaching programming, which we follow for Hedy too. This allows us to focus the full attention of the learner on the new syntactic element.
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Adding syntactic elements like brackets and colons is deferred to the latest moment possible:
Previous research in the computer science education domain has shown that operators such as == and : can be especially hard for novices. In a study with high-schoolers we found that that might be due to their pronunciation.
Research from natural language acquisition also indicates that parentheses and the colon are among the latest element of punctuation that learners typically learn. Given the choice between colons and parenthesis and other elements like indentation, the latter are introduced first.
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Learning new forms is interleaved between concepts as much as possible:
We know that spaced repetition is a good way of memorizing, and that it takes time to learn punctuation, so we give students as much opportunity as possible to work with concepts before syntax changes.
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At every level it is possible to create simple but meaningful programs:
It is important for all learners to engage in meaningful activities. Our experience in teaching high-school students (and even university CS students) is that learning syntax is not always seen as a useful activity. Students experience a large discrepancy between the computer being smart, for example by being able to multiply 1,910 and 5,671 within seconds, while simultaneously not being able to add a missing colon independently. We anticipate that when the initial syntax is simple, allowing novices to create a fun and meaningful program, they will later have more motivation to learn the details of the syntax.
In its current form, Hedy consists of 13 different levels. The levels loosely follow the lesson series Python in de klas ("Python in the classroom") in such a way that these existing lessons can be executed with Hedy instead of with Python.
At the first level, students can firstly print text. For this, no
syntactic elements are needed other than the keyword [print]
followed by arbitrary text. Furthermore students can ask for
input of the user using the keyword [ask]. Here we decided to use
the keyword [ask] rather than [input] because it is more aligned
with what the role of the keyword is in the code than with what it
does. Input of a user can be repeated with [echo], so very simple
programs can be created in which a user is asked for a name or a
favorite animal, fulfilling Design Goal 6.
At the second level, variables are added to the syntax. Defining a
variable is done with the word [is] rather than the equals symbol
fulfilling Design Goal 3 and Design Goal 4. We also add the option to
create lists and retrieve elements, including random elements from lists
with [at]. Adding lists and especially adding the option to select a
random item from a list allows for the creation of more interesting
programs such as a guessing game, a story with random elements, which is
an assignment from Python in de klas
("Python in the classroom"), or a customized dice.
In Level 3 the first syntactic element is introduced: the use of
quotation marks to distinguish between strings and text. In teaching
novices we have seen that this distinction can be confusing for a long
time, so offering it early might help to draw attention to the fact that
computers need information about the types of variables. This level is
thus an interesting combination of explaining syntax and explaining
programming concepts, which underlines their interdependency. The
variable syntax using [is] remains unchanged, meaning that learners
can now use both [number is 12] and [name is Hedy].
In Level 5, selection with the if statement is introduced, but the
syntax is ‘flat’, i.e. placed on one line, resembling a regular syntax
more:
[if name is print ]
Else statements are also included, and are also placed on one line,
using the keyword [else]:
[if name is print else print ].
In working with non-English native Python novices. Research has found
the keyword [for] to be a confusing word for repetition, especially
because it sounds like the word ‘four’ [@hermans_code_2018]. For our
first simplest form, according to Design Goal 2, we opt to use
Quorum [@stefik_quorum_2017] syntax [repeat x times]. In this
initial form, like the if the syntax is placed on one line:
[repeat 5 times print ]
Note: this level was originally planed as 4, is now moved up to 6. Might be moved to another place based on experiences of kids testing now.
In Level 4, students learn to calculate with variables, so addition,
multiplication, subtraction and division are introduced. While this
might seem like a simple step, our experience taught us that the use of
[*] for multiplication, rather than
After Level 6, there is a clear need to ‘move on’, as the body of a loop (and also that of an if) can only consist of one line, which limits the possibilities of programs that users can create. We assume this limitation will be a motivating factor for learners, rather than ‘having to learn’ the block structure of Python, they are motivated by the prospect of building larger and more interesting programs (Design Goal 6). The syntax of the loop remains otherwise unchanged as per Design Goal 3, so the new form is:\
[repeat 5 times]
[print ]
[print ]\
Once blocks are sufficiently automatized, learners will see a more
Python-like form of the for loop, namely: [for i in range 0 to 5].
This allows for access to the loop variable [i] and this allows for
more interesting programs, such as counting to 10. As per Design Goal 3,
the change is made small, and to do so (following Design Goal 4),
brackets and colons are deferred to a later level, but indentation which
was learned in Level 7 remains.
To make the step to full Python, learners will need to use the colon to denote the beginning of a block, in both loops and conditionals. Because blocks are already known, we can teach learners to use a colon before every indentation, and have them practice that extensively.
To allow for enough interleaving of concepts (Design Goal 5), we defer the introduction of round brackets and focus on more conceptual additions: the nesting of blocks. We know indentation is a hard concept for students to learn, so this warrants its own level (Design Goal 3).
Level 11 adds round brackets in [print], [range] and
[input]. As per Design Goal 4, these are added as late as possible.
In level 12, learners encounter different types of brackets for the
first time, because it adds rectangular brackets for list access, which
up to now was done with the keyword [at], following Design Goal 2.
In the final level Hedy becomes a subset of Python by replacing the word
[is] in assignment and equality checks by [=] and [==].