Day 5: Creating INI Parser

this is a pure Ini parser for nim

Nim has advanced parsecfg

What to expect ?

let sample1 = """

appname = configparser
version = 0.1

name = xmonader
email =


var d = parseIni(sample1)

# doAssert(d.sectionsCount() == 2)
doAssert(d.getProperty("general", "appname") == "configparser")
doAssert(d.getProperty("general","version") == "0.1")
doAssert(d.getProperty("author","name") == "xmonader")
doAssert(d.getProperty("author","email") == "")

d.setProperty("author", "email", "")
doAssert(d.getProperty("author","email") == "")
doAssert(d.hasSection("general") == true)
doAssert(d.hasSection("author") == true)
doAssert(d.hasProperty("author", "name") == true)
d.deleteProperty("author", "name")
doAssert(d.hasProperty("author", "name") == false)

echo d.toIniString()
let s = d.getSection("author")
echo $s


You can certainly use regular expressions, like pythons configparser, but we will go for a simpler approach here, also we want to keep it pure so we don't depend on pcre

Ini sample

appname = configparser
version = 0.1

name = xmonader
email =

Ini file consists of one or more sections and each section consists of one or more key value pairs separated by =

Define your data types

import tables, strutils

We will use tables extensively

type Section = ref object
    properties: Table[string, string]

Section type contains properties table represents key value pairs

proc setProperty*(this: Section, name: string, value: string) =[name] = value

To set property in the underlying properties table

proc newSection*() : Section =
    var s = Section() = initTable[string, string]()
    return s

To create new Section object

proc `$`*(this: Section) : string =
    return "<Section" & $ & " >"

Simple toString proc using $ operator

type Ini = ref object
    sections: Table[string, Section]

Ini type represents the whole document and contains a table section from sectionName to Section object.

proc newIni*() : Ini = 
    var ini = Ini()
    ini.sections = initTable[string, Section]()
    return ini

To create new Ini object

proc `$`*(this: Ini) : string = 
    return "<Ini " & $this.sections & " >"

define friendly toString proc using $ operator

Define API

proc setSection*(this: Ini, name: string, section: Section) =
    this.sections[name] = section

proc getSection*(this: Ini, name: string): Section =
    return this.sections.getOrDefault(name)

proc hasSection*(this: Ini, name: string): bool =
    return this.sections.contains(name)

proc deleteSection*(this: Ini, name:string) =

proc sectionsCount*(this: Ini) : int = 
    echo $this.sections
    return len(this.sections)

Some helper procs around Ini objects for manipulating sections.

proc hasProperty*(this: Ini, sectionName: string, key: string): bool=
    return this.sections.contains(sectionName) and this.sections[sectionName].properties.contains(key)

proc setProperty*(this: Ini, sectionName: string, key: string, value:string) =
    echo $this.sections
    if this.sections.contains(sectionName):
        this.sections[sectionName].setProperty(key, value)
        raise newException(ValueError, "Ini doesn't have section " & sectionName)

proc getProperty*(this: Ini, sectionName: string, key: string) : string =
    if this.sections.contains(sectionName):
        return this.sections[sectionName].properties.getOrDefault(key)
        raise newException(ValueError, "Ini doesn't have section " & sectionName)

proc deleteProperty*(this: Ini, sectionName: string, key: string) =
    if this.sections.contains(sectionName) and this.sections[sectionName].properties.contains(key):
        raise newException(ValueError, "Ini doesn't have section " & sectionName)

More helpers around properties in the section objects managed by Ini object

proc toIniString*(this: Ini, sep:char='=') : string =
    var output = ""
    for sectName, section in this.sections:
        output &= "[" & sectName & "]" & "\n"
        for k, v in
            output &= k & sep & v & "\n" 
        output &= "\n"
    return output

Simple proc toIniString to convert the nim structures into Ini text string


OK, here comes the cool part

Parser states

type ParserState = enum
    readSection, readKV

Here we have two states

  • readSection: when we are supposed to extract section name from the current line
  • readKV: when we are supposed to read the line in key value pair mode

ParseIni proc

proc parseIni*(s: string) : Ini = 

Here we define a proc parseIni that takes a string s and creates an Ini object

    var ini = newIni()
    var state: ParserState = readSection
    let lines = s.splitLines
    var currentSectionName: string = ""
    var currentSection = newSection()
  • ini is the object to be returned after parsing
  • state the current parser state (weather it's readSection or readKV)
  • lines input string splitted into lines as we are a lines based parser
  • currentSectionName to keep track of what section we are currently in
  • currentSection to populate ini.sections with Section object using setSection proc
   for line in lines:

for each line

         if line.strip() == "" or line.startsWith(";") or line.startsWith("#"):

We continue if line is safe to igore empty line or starts with ; or #

        if line.startsWith("[") and line.endsWith("]"):
            state = readSection

if line startswith [ and ends with ] then we set parser state to readSection

        if state == readSection:
            currentSectionName = line[1..<line.len-1]
            ini.setSection(currentSectionName, currentSection)
            state = readKV

if parser state is readSection

  • extract section name between [ and ]
  • add section object to the ini under the current section name
  • change state to readKV to read key value pairs
  • continue the loop on the nextline as we're done processing the section name.
        if state == readKV:
            let parts = line.split({'='})
            if len(parts) == 2:
                let key = parts[0].strip()
                let val = parts[1].strip()
                ini.setProperty(currentSectionName, key, val)

if state is readKV

  • extract key and val by splitting the line on =
  • setProperty under the currentSectionName using key and val
    return ini

Here we return the populated ini object.