haskell - Unwrapping the STT monad in a transformer stack? -

this question apparently related problem discussed here , here. unfortunately, requirement different questions, , answers given don't apply me. don't understand why runst fails type check in these cases, doesn't help.

my problem this, have 1 section of code uses 1 monad stack, or rather single monad:

import control.monad.except type kerr = except kinderror 

another section of code needs integrate wraps within stt monad:

type runm s = stt s (except kinderror) 

at interface between these sections, need wrap , unwrap outer layer. i've got following function work in kerr -> runm direction:

kerrtorun :: kerr -> runm s kerrtorun e = either throwerror return $ runexcept e 

but reason can't converse type check:

runtokerr :: runm s -> kerr runtokerr r = runst r 

i'm working on assumption runm's inner monad has same structure kerr, can return once i've unwrapped stt layer, don't seem able far, runst complains type arguments:

src/kindlang/runtime/eval.hs:18:21:     couldn't match type ‘s’ ‘s1’       ‘s’ rigid type variable bound           type signature runtokerr :: runm s -> kerr           @ src/kindlang/runtime/eval.hs:17:14       ‘s1’ rigid type variable bound            type expected context:              stt s1 (exceptt kinderror data.functor.identity.identity)            @ src/kindlang/runtime/eval.hs:18:15     expected type: stt                      s1 (exceptt kinderror data.functor.identity.identity)       actual type: runm s 

i've tried:

runtokerr r = either throwerror return $ runexcept $ runst r 

in order more isolate runst expected return type, in case cause of issue, results same.

where s1 type coming from, , how persuade ghc it's same type s?

(the below talks st s a applies same stt s m a; i've avoided unnecessary complication of talking transformer version below)

the problem seeing runst has type (forall s. st s a) -> a isolate potential (stref-changing) effects of computation outside, pure world. whole point of s phantom type st computations, strefs, etc. tagged with, track "st-domain" belong to; , type of runst ensures nothing can pass between domains.

you can write runtokerr enforcing same invariant:

{-# language rank2types #-}  runtokerr :: (forall s. runm s a) -> kerr runtokerr = runst 

(of course, might realize further down road restriction strong program hope write; @ point you'll need lose hope, sorry, mean you'll need redesign program.)

as error message, reason can't "convince type checker s1 , s same type" because if pass st s a given choice of s , a, not same giving allows choose own s. ghc chose s1 (a skolemized variable) s , tries unify st s a st s1 a