lenses - Why can't I bind and reuse a haskell lens like a normal variable? -

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(i'm using lens-family, not lens)

i have deep data structure , need focus on 2 parts have common path. intuitively define _table intention reuse it:

let _table = _soutput.at (o.tname table)._just' tp' <- evalcacheable (_table.o._stperm) m.empty ... reuse _table

but yields error:

could not deduce (functor f0) arising use of ‘_soutput’ context (monadreader (evalconf state all) m, ...

yet, directly pasting value of _table argument works:

tp' <- evalcacheable (_soutput.at (o.tname table)._just'.o._stperm) m.empty

what going on? can give more details types involved, me seems puzzling regardless. under impression that

let x = y z <- f x

was equivalent to

z <- f y

in cases.

this related monomorphism restriction. try putting {-# language nomonomorphismrestriction #-} @ top of file. –randomusername

it worked! on #haskell solved adding explicit type signature _table. according him rank2types messing type inference. – bruceberry


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