2.1.1 Primitives and Literals

There are several different literal token types referred to in this documentation.

2.1.1.1 Names

Names in Pyret match the following regular expression:

^[_a-zA-Z][_a-zA-Z0-9]*(?:-+[_a-zA-Z0-9]+)*

a
a1
a-1
abc
ABC
a----------b
a-_-_-_-__--b
a--_
_a
__

The following are not valid names:

_-
-_
a-
-a
-a-
-abc
a1-
a-1-
a_1-
α
1abc
$abc

2.1.1.2 String Literals

‹string-expr›: STRING

Strings in Pyret come in several forms. First, they can be enclosed in double quotes:

"a string"
"a string\" with escapes"
"'single quotes' are allowed unescaped or \' escaped"

They can also be enclosed in single quotes:

'a string'
'a string\' with escapes'
'"double quotes" are allowed unescaped or \" escaped'

String literals with single or double quotes must terminate by the end of the line:

"multi-line
strings not
allowed with double quotes"

Finally, multi-line string literals can be created by starting and ending them with three backticks (“‘). For example:

```
This string
spans
multiple lines
```

Multi-line string literals strip all whitespace before the first non-whitespace character and after the last non-whitespace character. All whitespace at the beginning of intermediate lines is preserved.

2.1.1.3 Number Literals

‹num-expr›: NUMBER

Pyret has several types of number literals. The most traditional allows for decimal numbers, negation, and an exponent:

^[-+]?[0-9]+(?:\\.[0-9]+)?(?:[eE][-+]?[0-9]+)?

That is, an optional sign, then some number of digits, optionally followed by a decimal point and more digits, optionally followed by an exponent. These are valid number literals:

0.1
1
1e100
1.1e100
+1.1e100
-1.1e-100
1.1230e-0
10
19
19.0

Note that a number literal cannot start with a decimal point; some leading digits are required. These are not number literals:

.1
1.1.1
1.+1
0.1+100

This first kind of number literal represents an exact number, or Exactnum. Number literals can also be prefixed with a tilde, to indicate that the number is an approximation, or a Roughnum. So these are all valid rough number literals:

~0.1
~1
~1e100
~1.1e100
~+1.1e100
~-1.1e-100
~1.1230e-0
~10
~19
~19.0

And these are not valid:

~.1
~1.1.1
~1.+1
~0.1+100

Finally, numbers can be written as exact ratios of whole numbers:

^[-+]?[0-9]+/[0-9]+

These numbers are interpreted as Exactnums. These are valid rational literals:

1/2
-1/2
+1/4
1234/9
0/1234

It is a syntax error to use zero as the denominator in a fraction literal. These are not valid rational literals:

1+1/2
-1/0
1.1/9
1/-3

2.1.1.4 Boolean Literals

‹bool-expr›: true | false

Boolean literals are the lowercase words true and false.