MJr

Types

MJr is a statically-typed language; each expression and variable has a type, and expressions are type-checked at compile-time. This allows compilation to statically-typed target languages, including TypeScript. However, all types in MJr are inferred, so type annotations are never needed (and the language has no syntax for them anyway).

Primitive types

The following types are primitive:

• bool: Boolean
• float: double-precision floating-point number
• fraction: exact rational number
• grid
• int: signed 32-bit integer
• str: string

There are no subtype relations between any primitive types.

Dict types

Dicts are immutable unordered collections of key/value pairs; a dict type is the type of such a collection, i.e. an unordered set of key names, each associated with a value type. There are no “dynamic” dictionary types; every dict type has a fixed set of keys.

A dict type D1 is a subtype of a dict type D2 if D1 and D2 have the same keys, and each key’s value type in D1 is a subtype of its value type in D2.

Note that a dict type with more keys than another dict type is not a subtype. This allows easier compilation to a wider range of target languages.

Pattern types

A pattern.in or pattern.out type represents an immutable rectangular pattern which may occur in a grid, or which may be written to a grid, respectively. A pattern type consists of an alphabet, a pattern width, a pattern height.

A pattern type P1 is a subtype of a pattern type P2 if P1 and P2 are equal, or if P1 is a pattern.out type with the same alphabet, width and height as P2.

Position types

A position type represents an immutable pair of x and y coordinates for a cell in a grid. The coordinates (0, 0) refer to the top-left corner. A position type is particular to some grid; position types are distinct (and not related by subtypes) if they are for different grids, even if those grids have the same alphabet and scale.

Type coercion

MJr allows two kinds of type coercions.

Coercion from ‘int’

Expressions of type int can be coerced to float or fraction. This coercion occurs:

• Anywhere that a float or fraction is required;
• In any binary operator expression where the other operand has type float or fraction; or
• In a branch of a conditional expression, where the other branch has type float or fraction.

Coercion to str

Expressions of type bool, float, fraction, grid or int can be coerced to str. This coercion occurs:

• In a log statement;
• In a binary + expression where the other operand has type str; or
• In a branch of a conditional expression, where the other branch has type str.

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