Phillip Trelford's Array

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Basic Tuples & Pattern Matching

Over the last couple of weeks I’ve been building my own parser, interpreter and compiler for Small Basic, a dialect of BASIC with only 14 keywords aimed at beginners. Despite, or perhaps because of, Small Basic’s simplicity some really fun programs have been developed, from games like Tetris and 3D Maze to a parser for the language itself.

Small Basic provides primitive types for numbers, strings and associative arrays. There is no syntax provided for structures, but these can be easily modelled with the associative arrays. For example a 3D point can be constructed with named items or ordinals:

Named items Ordinals
Point["X"] = 1.0
Point["Y"] = 2.0
Point["Z"] = 3.0
Point[0] = 1.0
Point[1] = 2.0
Point[2] = 3.0

In languages like Erlang and Python this could be more concisely expressed as a tuple:

Erlang Python
Point = {1.0, 2.0, 3.0}
point = (1.0, 2.0, 3.0)

In fact sophisticated Erlang programs are built entirely from tuples and lists, there is no explicit class or inheritance syntax in the language. Messages can be easily expressed with tuples and behaviour via pattern matching.

Alan Kay, inventor of the Smalltalk language has said:

The notion of object oriented programming is completely misunderstood. It's not about objects and classes, it's all about messages.

In Erlang a hierarchy of shapes can simply be modelled using tuples with atoms for names:

Circle = { circle, 5.0 }
Square = { square, 7.0 }
Rectangle = { rectangle, 10.0, 5.0 }

The area of a shape can be expressed using pattern matching:

area(Shape) ->
  case Shape of
    { circle, R } -> pi() * R * R;
    { square, W } -> W * W;
    { rect, W, H } -> W * H

Select Case

The Visual Basic family’s Select Case functionality is quite rich. More so than the switch/case statements of the mainstream C dialects: Java, C# and C++, which only match literals.

In Visual Basic it is already possible to match values with literals, conditions or ranges:

Select Case agerange
  Case Is < 16
  Case 16 To 21
    MsgBox("Still Young")
  Case 50 To 64
    MsgBox("Start Lying")
  Case Is > 65
    MsgBox("Be Yourself") 
  Case Else
End Select

Given that Select Case in VB is already quite expressive, it feels support for tuples and pattern matching over them would feel quite natural in the language.

Extended Small Basic

To this end I have extended my Small Basic parser and compiler implementation with tuple and pattern matching support.


Inspiration for construction and deconstruction was taken from F# and Python:

F# Python
let person = ("Phil", 27)
let (name, age) = person
person = ("Phil", 27)
name, age = person

So that tuples use explicit parentheses in the extended Small Basic implementation:

Person = ("Phil", 27)
(Name, Age) = Person

Internally tuples are represented using Small Basic’s built-in associative arrays.

Pattern Matching

First I implemented VB’s Select Case statements, which is not hugely dissimilar to parsing and compiling Small Basic’s If/ElseIf/Else statements.

Then I extended Select Case to support matching tuples with similar functionality to F#:

F# Extended Small Basic
let xor x y =
  match (x,y) with
  | (1,1) -> 0
  | (1,0) -> 1
  | (0,1) -> 1
  | (0,0) -> 0
Function Xor(a,b)
  Select Case (a,b)
    Case (1,1)
      Xor = 0
    Case (1,0)
      Xor = 1
    Case (0,1)
      Xor = 1
    Case (0,0)
      Xor = 0

Constructing, deconstructing and matching nested tuples is also supported.


Putting it altogether, FizzBuzz can now be expressed in my extended Small Basic implementation with functions, tuples and pattern matching:

Function Mod(Dividend,Divisor)
  Mod = Dividend
  While Mod >= Divisor
    Mod = Mod - Divisor

Sub Echo(s)

For A = 1 To 100 ' Iterate from 1 to 100
  Select Case (Mod(A,3),Mod(A,5))
    Case (0,0)
    Case (0,_)
    Case (_,0)
    Case Else


Extending Small Basic with first class support for tuples was relatively easy, and I feel quite natural in the language. It provides object orientated programming without the need for a verbose class syntax. I think this is something that would probably work pretty well in other BASIC dialects including Visual Basic.

Source code is available on BitBucket:

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