Empty Show instance + phantom type + GHCi = out of memory

[japple]

module ShowMemory where

data M a = M

instance Show a => Show (M a)

Load in GHCi, type "M" and then enter at the prompt.

It kills all my memory. I have no .ghci

[pumpkin]

I get the same behavior on Mac OS 10.5, using 6.12.1, also no .ghci.

[pumpkin]

This is just like doing

instance Eq M

and having a call to (==) loop between (/=) and (==) infinitely. show calls shows which calls showsPrec which calls show. I don't think this is a bug.

[igloo]

pumpkin is right: The default Show instance includes

show x          = shows x ""
showsPrec _ x s = show x ++ s

and

shows           =  showsPrec zeroInt

So we have

show M = shows M "" = showsPrec zeroInt M "" = show M ++ ""