[Haskell-cafe] Why does this result in a runtime loop instead of a type error or typechecker loop?

Anthony Clayden anthony_clayden at clear.net.nz
Wed Jul 25 23:02:40 UTC 2018

On Tue, Jul 24, 2018 at 5:52 AM,Ryan Reich wrote:
> This seems like the canonical illustration of the dangers of UndecidableInstances.

No I don't buy that explanation. The worst UndecidableInstances can do
is send the type checker into a loop.

For mutually recursive functions to cause looping, you don't need
classes at all:

foo :: Bool -> ()
foo x = bar x
bar :: () -> Bool
bar x = foo x

No typechecking will detect that loop. This `HasBool` example is just
an obfuscated way to write that loop(?)

There's some odd thigs in the OP:

Why use OVERLAPPABLE? Neither of those instances need it.

Or are there more instances in the imports? (There must at least be
the definition for class `Has`.) If so, what are their constraints?
And what are their implementations? Perhaps that's causing the

Will `runRIO` or `view` use one of those other instances?

What does this mean: "I can compile an equivalent program; it loops on

There doesn't need to be a `Bool` in the "environment": the `HasBool
a` instance introduces `Bool` in its constraint.

I suggest boiling this down to a stand-alone program not needing
imports. Then tell the whole story.


On Mon, Jul 23, 2018 at 2:10 PM, Theodore Lief Gannon <
tanuki at gmail.com

>* λ. :set -XFlexibleContexts -XFlexibleInstances -XMultiParamTypeClasses
*>* -XUndecidableInstances
*>* λ. import RIO
*>* λ. import Data.Has
*>* λ. class HasBool a where boolL :: Lens' a Bool
*>* λ. instance {-#OVERLAPPABLE#-} HasBool a => Has Bool a where hasLens =
*>* boolL
*>* λ. instance {-#OVERLAPPABLE#-} Has Bool a => HasBool a where boolL =
*>* hasLens
*>* λ. runRIO () $ view boolL
*>* ^CInterrupted.
*>>* The RIO environment () doesn't contain a Bool. I can see how the
*>* typechecker might get lost in this, but I can compile an equivalent
*>* program; it loops on execution.
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