[Haskell-cafe] Best way to return Bool based on a successful pattern match?

[Nikolay Amiantov]

Very interesting trick, thanks, it feels closer to what I've imagined!

Meanwhile I've played with quasi-quoters and TH a bit for the sake of
experiment. As a result I got this:

https://github.com/abbradar/isit/blob/master/src/Language/Haskell/IsIt.hs

It allows one to do very nice-looking checks, like:

λ filter [is|Left (Just 4)|] [Right "test", Left (Just 4), Left Nothing]
[Left (Just 4)]
λ let x = 4
λ filter [is|Left (Just x)|] [Right "test", Left (Just 4), Left Nothing]
[Left (Just 4)]
λ :t [is|Just _|]
[is|Just _|] :: Maybe t -> Bool

I doubt anyone including me would actually use this though -- it depends
on haskell-src-{exts,meta} which you probably don't want as yet more
dependencies for your project to do such a trivial task.

On 07/13/2015 05:06 PM, Erik Hesselink wrote:
> You can write a generic 'is constructor' function using GHC Generics,
> if you want:
> 
> {-# LANGUAGE
>     DeriveGeneric
>   , FlexibleContexts
>   , FlexibleInstances
>   , FunctionalDependencies
>   , MultiParamTypeClasses
>   , UndecidableInstances
>   #-}
> 
> import GHC.Generics
> import Generics.Deriving.ConNames
> 
> class SameType a b | b -> a where
>   saturate :: b -> a
> 
> instance SameType a a where
>   saturate = id
> 
> instance SameType a b => SameType a (c -> b) where
>   saturate f = saturate (f undefined)
> 
> is :: (ConNames (Rep a), Generic a, SameType a b) => b -> a -> Bool
> is ctor val = conNameOf val == conNameOf (saturate ctor)
> 
> Now you can do things like:
> 
> data Foo = A | B Int
>   deriving (Show, Generic)
> 
> *Main> is A A
> True
> *Main> is A (B 1)
> False
> *Main> is B A
> False
> *Main> is B (B 2)
> True
> 
> Erik
-- 
Nikolay.