[Haskell-beginners] Using state inside quickcheck generators

[Kyle Murphy]

I myself am fairly new to Haskell and I'm trying to understand Monads better
myself, but maybe this could help?
http://user.cs.tu-berlin.de/~magr/pub/Transformers.en.html
It's a little out of date and won't compile cleanly on newer instances of
GHC without changing all the instances of Map.lookup ... to return $
fromJust Map.lookup, but once you make that small change it works fine.
Also, I'm not familiar with the QuickCheck package, but I browsed through it
quickly and Test.QuickCheck.Monadic caught my eye. Perhaps a State or StateT
combined with IdM or PropertyM would do what you want?

On Tue, Sep 15, 2009 at 5:56 PM, Alexander MacDonald <alexmac at adobe.com>wrote:

> Hi guys, I¹ve been using quickcheck for a while and I¹m having no problem
> making generators for my data types, however recently I stumbled upon a
> situation where I need to maintain State inside the quickcheck generator
> functions. Here's a simple example, imagine I've got a basic language which
> only has two features, variable definitions and assignments:
>
> type Script = [Statement]
>
> data Name = Name String
>    deriving (Show,Eq)
>
> data Statement = Def Name Int | Assign Name Name
>    deriving Show
>
> Generating random scripts is easy enough, just make each type an instance
> of
> Arbitrary with a suitable generator function, however I want to make sure
> that the generated scripts are syntactically valid. In this case that means
> no "Name" can be used in an "Assign" statement unless it has previously
> been
> defined by a "Def" statement.
>
> My knowledge of using monads is somewhat basic so I'm struggling to figure
> out how to use the State monad with QuickCheck. I've pasted my current
> effort on the web: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=9412
>
> If anyone could spare a moment to glance at my code and give me a push in
> the right direction that would be much appreciated!
>
> Thanks,
> Alex Mac
>
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