| Safe Haskell | None |
|---|---|
| Language | Haskell2010 |
Text.Trifecta.Tutorial
Description
This module provides a short introduction to get users started using Trifecta. The key takeaway message is that it’s not harder, or even much different, from using other parser libraries, so for users familiar with one of the many Parsecs should feel right at home.
The source of this file is written in a literate style, and can be read top-to-bottom.
Documentation
importDocumentation :: docDummy Source #
First, we import Trifecta itself. It only the core parser definitions and
instances. Since Trifecta on its own is just the parser and a handful of
instances; the bulk of the utility functions is actually from a separate
package, parsers, that provides the usual parsing functions like
manyTill, between, and so on. The idea behind the parsers package is
that most parser libraries define the same generic functions, so they were
put into their own package to be shared. Trifecta reexports these
definitions, but it’s useful to keep in mind that the documentation of
certain functions might not be directly in the trifecta package.
In order to keep things minimal, we define a very simple language for arithmetic expressions.
parseExpr :: Parser Expr Source #
The parser is straightforward: there are literal integers, and parenthesized additions. We require parentheses in order to keep the example super simple as to not worry about operator precedence.
It is useful to use tokenizing functions to write parsers. Roughly
speaking, these automatically skip trailing whitespace on their own, so that
the parser isn’t cluttered with skipWhitespace calls. symbolic for
example parses a Char and then skips trailing whitespace; there is also the
more primitive char function that just parses its argument and nothing
else.
We can now use our parser to convert a String to an Expr,
parseString parseExpr mempty "(1 + (2 + 3))"
Success (Add (Lit 1) (Add (Lit 2) (Lit 3)))
When we provide ill-formed input, we get a nice error message with an arrow to the location where the error occurred:
parseString parseExpr mempty "(1 + 2 + 3))"
(interactive):1:8: error: expected: ")"
(1 + 2 + 3))<EOF>
^