Oblig 2: Parsing CSV

In this exercise you will use parser combinators to write elegant and powerful parsers. To start things off, you will write a simple CSV parser. We will then extend it with a simple type system, so that the data the parser returns is correctly typed. Finally we will add a few unary and binary operators (abs, max, min, add) to the parser, so that it can perform computation while parsing.

You will be given a stub project to work on. The project has two main parts: a parser combinator library and a stub csv parser. The parser combinator library is largely compatible with the production level FParsec library, but is much more simple minded (slow and incomplete). It is provided for reference, but you should probably use FParsec for the actual project, since the error reporting is much better, making debugging easier.

Prerequisistes

It is essential to watch Scott Wlaschin's excellent presentation on parser combinators before you begin. It's also helpful to go through his blog posts on the topic:

1. Video
2. Blog posts
3. Study the provided parser combinator library in Parsec.fs. It's very similar to Scott's library.

Exercises

1. Using the stub in src/Oblig2.fs, write a parser to parse the contents of ex1.csv as strings.

2. Modify the parser from 1. to return values with the correct type, e.g. ints as ints, floats as floats, etc.

   Hint: The Parser is a functor. Use map (or |>>) to convert the parsed string to the right type of value. You can transform the output from the parser to a more useful structure (record type) using a smart constructor and the applicative combinators <!> and <*>.

3. Improve the parser to first parse a header with the types of the columns, then parse each line according to the type specification.

   Hint: Using |>> you can change the parser from e.g Parser<string> to Parser<Parser<string>>. You can thus parse a type specification (e.g. "int"), and return a new parser of ints. The returned parser can be run to parse consequent lines of input, containing the actual csv data.

4. Modify the Parser from 2. to include the following typed operators: abs, add, max, min. The type is specified id the following manner: op : type, e.g. "add : int" (see ex3.csv).

5. Implement a computatin expression for the parser combinator, and implement 2. (or 3.) in "monadic" style using the computaton expression syntax.

Building the project

Prerequisites

• Install .NET 5.0

Building and running

cd src
dotnet build
dotnet run ex1.csv

Good luck

This exericse combines almost everything you have learned so far: Functions, higher-order functions, partial application, functors, applicative funtors, monads and custom operators. It's a mouthful, but you can do it. Talk to each other, and don't be afraid to ask questions on Canvas. I will answer to the best of by ability.

May the foo be with you!