Create shrinker for fixed length string that does not shrink to an empty string

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1

I have written a Fuzzer that generates fixed length alphanumeric (latin) strings. The generation works fine, but I cannot figure out how to apply a shrinker that generates a shrunk value that conforms to my length restriction and shrinks appropriately.

If I implement the Fuzzer like below it generates the appropriate string but shrinks incorrectly to empty string :

module HotFuzz exposing (..)

import Fuzz exposing (..)
import Random.Char as RandomChar
import Random.Extra as RandomExtra
import Shrink as Shrink exposing (Shrinker)

nLengthUpperCaseAlphanumeric: Int -> Fuzzer String
nLengthUpperCaseAlphanumeric length =
    let
       charGen =  RandomExtra.choices (RandomChar.char 48 57) [RandomChar.upperCaseLatin]
       gen = RandomString.string length charGen
       charShrinker = Shrink.atLeastChar '0'
       shrinker = Shrink.convert (String.fromList) (String.toList) (Shrink.list charShrinker)
    in
       custom gen shrinker

I’ve also tried some more advanced shrinking but it doesn’t do what I want.

What I want is to have is a Shrinker that demonstrates the following properties:

  • The shrunk string should shrink to a string of length "length"
  • The shrunk string should have each char in it shrunk to '0' (i.e. for size 3 ABB should shrink to 000)

I’ve also tried a more advanced (convoluted) Shrinker that I’m not sure even shrinks:

nLengthUpperCaseAlphanumeric : Int -> Fuzzer String
nLengthUpperCaseAlphanumeric length =
    let
       charGen =  RandomExtra.choices (RandomChar.char 48 57) [RandomChar.upperCaseLatin]
       gen = RandomString.string length charGen
       charShrinker = Shrink.atLeastChar '0'
       mkString ch = List.repeat length ch |> String.fromList
       shrinker str = 
            str
            |> String.toList
            |> List.map (\_ -> '0')
            |> List.head
            |> Maybe.withDefault '0'
            |> charShrinker
            |> Shrink.map mkString           
    in
       custom gen shrinker

This “shrinker” does keep my string length to the expected length, but it collapses to more choices than I’d expect.

1 Like
2

Shrink.list by default will shrink first to an empty list, which is not what you want, as you then get an empty string.

You can force it to shrink to a String of wanted length by adding |> Shrink.keepIf (\str -> String.length str == length) to your shrinker.

nLengthUpperCaseAlphanumeric : Int -> Fuzzer String
nLengthUpperCaseAlphanumeric length =
    let
        charGen =
            Random.Extra.choices (Random.Char.char 48 57) [ Random.Char.upperCaseLatin ]

        gen =
            Random.String.string length charGen

        charShrinker =
            Shrink.atLeastChar '0'

        charsShrinker chars =
            List.map charShrinker chars

        shrinker =
            Shrink.convert String.fromList String.toList (Shrink.list charShrinker)    
                |> Shrink.keepIf (\str -> String.length str == length)   
    in
    custom gen shrinker
1 Like
3

Thanks this works great. I guess I was mistaken in that I looked at keepIf as being a filter, but it actually kind of acts like a means of “pinning” a value.

Thanks gain.

4

I think your intuition was good, it is a filter, but for the lazy list of shrinked values to test.
So only the strings of expected length will be used as shrinked values to test.

PS: You could of course have also used |> Shrink.dropIf (\str -> String.length str /= length).

5

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