scala - How to extract an element from an HList with a specific (parameterized) type -

i'm chaining transformations, , i'd accumulate result of each transformation can potentially used in subsequent step, , results available @ end (mostly debugging purposes). there several steps , time time need add new step or change inputs step.

hlist seems offer convenient way collect results in flexible still type-safe way. i'd rather not complicate actual steps making them deal hlist , accompanying business.

here's simplified version of combinator i'd write, isn't working. idea given hlist containing a, , index of a, , function -> b, mapnth extract a, run function, , cons result onto list. resulting extended list captures type of new result, several of these mapnth-ified steps can composed produce list containing result each step:

def mapnth[l <: hlist, a, b]     (l: l, index: nat, f: => b)     (implicit at: shapeless.ops.hlist.at[l, index.n]):     b :: l =   f(l(index)) :: l 

incidentally, i'll need map2nth taking 2 indices , f: (a, b) => c, believe issues same.

however, mapnth not compile, saying l(index) has type at.out, f's argument should a. that's correct, of course, suppose need way provide evidence at.out in fact a (or, at.out <: a).

is there way express constraint? believe have take form of implicit, because of course constraint can checked when mapnth applied particular list , function.

you're right needing evidence at.out a, , can provide evidence including value of type member in at's type:

def mapnth[l <: hlist, a, b]     (l: l, index: nat, f: => b)     (implicit at: shapeless.ops.hlist.at[l, index.n] { type out = }):     b :: l =   f(l(index)) :: l 

the companion objects type classes at in shapeless define aux type includes output type final type parameter.

def mapnth[l <: hlist, a, b]     (l: l, index: nat, f: => b)     (implicit at: shapeless.ops.hlist.at.aux[l, index.n, a]):     b :: l =   f(l(index)) :: l 

this pretty equivalent more idiomatic (and looks little nicer).