customized comparisons for output

[aavogt]

I use doctest to check that some types can be properly inferred. I would like some way to customize the comparison so that the following tests would be accepted:

This fails because a isn't b... though they mean exactly the same thing:

{- |
>>>  let p = (+) :: Num b => b -> b -> b
>>> :t p
p :: Num a => a -> a -> a
-}

This fails because of whitespace.... sometimes ghc breaks type signatures
into different lines

{- |
>>>  let p = (+) :: Num b => b -> b -> b
>>> :t p
p :: Num a =>
             a -> a -> a
-}

As far as I know, there is no way to get a String in ghci out of :type, so I can't even do something confusing like:

{- |
>>> let x = show (1 + 1) ++ " next"
>>> let canonicalize = filter (/=' ')
>>> let expectedX = "2next"
>>> canonicalize x == canonicalize expectedX
True
-}

One idea might be to make the comparison happen in the ghci session using whatever == is in scope (or some other/better function name). In other words,

{- |
>>> let (==) a b = canonicalize a Prelude.== canonicalize b
>>> 1+1
2
<BLANKLINE>
3
-}

will send two lines to ghci like:

let (==) a b  = canonicalize a Prelude.== canonicalize b
show (1+1) == "2\n3"

[sol]

Not sure what to do here. My first impression is "too much hassle for no gain". But then again, what may be no gain for me, may be important to you. So if you really feel like hacking on this, go for it, but be prepared that we bikeshed it to death if it gets ugly..