Parse decimal number using Cuerdas in a ClojureScript program - parsing

Question

I am using Cuerdas library in a
ClojureScript program to parse floating point numbers. Sometimes the input from
a user is missing the integral part, like .1 for 0.1. I need to handle this.
But I ran into strange results when comparing the parsed numbers:
(= 0.1 (parse-number "0.1"))
;; => true
(= .1 (parse-number "0.1"))
;; => true
(= 0.1 (parse-number ".1"))
;; => false
(= .1 (parse-number ".1"))
;; => false
The last two results above are surprising to me. When comparing literal number I
get the expected result:
(= 0 .0)
;; => true
The strange behavior is also visible when I just parse the decimal number without integer
part, like here:
(parse-number ".1")
;; => .1
.1
;; => 0.1
I found out that I can use js/parseFloat to get what I want, but I'm curious
what is the reason for this behavior when using Cuerdas? What is this .1 value
and how is it different than 0.1?

Answer 1

parse-number source looked like this:
(defn parse-number
"General purpose function for parse number like
string to number. It works with both integers
and floats."
[s]
(if (nil? s)
#?(:cljs js/NaN :clj Double/NaN)
(if (numeric? s)
(edn/read-string s)
#?(:cljs js/NaN :clj Double/NaN))))
Both ".1" and "0.1" are considered to be numeric? (was implemented in cuerdas by a regex check):
cljs.user=> (def re #"^[+-]?([0-9]*\.?[0-9]+|[0-9]+\.?[0-9]*)([eE][+-]?[0-9]+)?$")
#'cljs.user/re
cljs.user=> (boolean (re-matches re "0.1"))
true
cljs.user=> (boolean (re-matches re ".1"))
true
so they will be read by cljs.reader/read-string.
When reading the string "0.1" the return type is a js/Number, but ".1" is of type cljs.core/Symbol:
cljs.user=> (cljs.reader/read-string ".1")
.1
cljs.user=> (cljs.reader/read-string "0.1")
0.1
cljs.user=> (type (cljs.reader/read-string "0.1"))
#object[Number]
cljs.user=> (type (cljs.reader/read-string ".1"))
cljs.core/Symbol
So although it looks like it correctly parsed the .1 it actually turned it into a symbol. The symbol .1 is not equal to the number .1.
Note that parse-number is no longer available in newer versions of cuerdas because it "is a string manipulation library not a numbers parsing library".

Answer 2

I have some numeric parsing functions that work for both CLJ and CLJS. From the unit tests, we have
The headers:
(ns tst.tupelo.parse
;---------------------------------------------------------------------------------------------------
; https://code.thheller.com/blog/shadow-cljs/2019/10/12/clojurescript-macros.html
; http://blog.fikesfarm.com/posts/2015-12-18-clojurescript-macro-tower-and-loop.html
#?(:cljs (:require-macros
[tupelo.misc]
[tupelo.testy]))
(:require
[clojure.test] ; sometimes this is required - not sure why
[tupelo.parse :as tpar]
[tupelo.misc :as misc]
[tupelo.core :as t :refer [spy spyx spyxx spyx-pretty]]
[tupelo.testy :refer [deftest testing is dotest isnt is= isnt= is-set= is-nonblank=
throws? throws-not? define-fixture]])
#?(:clj (:import [java.lang Math]))
)
Parsing integers:
#?(:cljs
(do
(dotest
(is= 0 (tpar/parse-int "0"))
(is= 15 (tpar/parse-int "15"))
(is= -5 (tpar/parse-int "-5"))
(is= 99999 (tpar/parse-int "99999"))
(throws? (tpar/parse-int ""))
(throws? (tpar/parse-int "05"))
(throws? (tpar/parse-int "123xxx"))
(throws? (tpar/parse-int "12x3"))
(throws? (tpar/parse-int "12.3"))
(throws? (tpar/parse-int "xxx123")))
Parsing floats
(dotest
(is= 0 (tpar/parse-float "0"))
(is= 0 (tpar/parse-float "0.0"))
(is= 12.345 (tpar/parse-float "12.345"))
(is= -5.1 (tpar/parse-float "-5.1"))
(is= 42 (tpar/parse-float "42.0"))
(is= 42 (tpar/parse-float "42"))
(is= 123.45 (tpar/parse-float "1.2345e2"))
(throws? (tpar/parse-float ""))
(throws? (tpar/parse-float "xxx1.23"))
(throws? (tpar/parse-float "1.23xxx"))
(throws? (tpar/parse-float "1.2xx34")))
))