Adventure in parserland – parsing lambda expressions in F# – Part II


Tags: fsharp Lambda expressions Parsing

The parser starts simple with the following two functions to parse either a string or a file. I use the XXX_Readers_ because I want to lazy read character by character.

let parseString s =
    let reader = new StringReader(s)
    parseTextReader reader
let parseFile fileName =
    let reader = new StreamReader(fileName: string)
    parseTextReader reader

The whole parser is in the following two lines:

let parseTextReader: TextReader -> seq<Expression> =
                    textReaderToLazyList >> tokenStream >> parseExpressions

I need to specify the signature otherwise the compiler gets confused : wait, does it take a StringReader or a StreamReader? You better tell me!

The function is a composite of three functions applied in sequence:

  1. Translate a TextReader to a LazyList
  2. Translate a LazyList to a LazyList (lexer)
  3. Translate a LazyList to a LazyList (parser)

My usage of LazyList as the workhorse for the program is because I want to match on the head of the stream of chars/tokens in a lazy way.

I love it when a program naturally decomposes in such simple understandable pieces. I impute some of that to functional programming. For one reason or another, in my 15+ years of object oriented programming, I’ve rarely got to the core of a problem with such immediacy.

A sequence of operations likes the above would be lost in a protected overridden implementation of a base class somewhere (or something else equally long to pronounce). The beauty would be lost somewhere in the vast machinery required to support it.

In any case, TextReaderToLazyList is a trivial generator function that uses the unfold function of LazyList to read a character at the time.

let textReaderToLazyList textReader = LazyList.unfold (fun (ts:TextReader) ->
    let ch = ts.Read()
    if ch = -1 then None else Some(char ch, ts)) textReader

The next step is to look at either the lexer, going bottom up, or the parser, going top down.

comments powered by Disqus Updated: 2020-02-06 14:56:41 +0100 +0100, Version: e6a6e8e