A function that sets an implicit parameter context

Question

I've been mucking around with implicit parameters which I've sometimes found useful, but trying to do something like the code below (which doesn't compile)

{-# LANGUAGE ImplicitParams #-}

main = print (f z)

f g =
  let 
    ?x = 42
    ?y = 5
  in
    g

z :: (?x :: Int) => Int
z = ?x

What I want basically is a function f which runs a given function with a "context". In the case above, f z will run z with ?x = 42, which naturally just returns 42 in this case and hence this program should print 42. But I get the error:

• Unbound implicit parameter ?x::Int arising from a use of ‘z’
• In the first argument of ‘f’, namely ‘z’
  In the first argument of ‘print’, namely ‘(f z)’
  In the expression: print (f z)

This is because there's no context for z where it's first used in main, even though f is providing the context.

The reason I'm attempting to do this is because I've got a number of functions which work with implicit parameters, and a GADT which has differing but similar implementation options. It's relatively trivial to write a function which extracts the appropriate implicit parameters from each of the possible GADT constructors, but then I want to apply a function with those implicit parameters.

Something like this approach (that compiles) would be good, or alternatively a different approach which allows me to easily set an implicit parameter context for a variety of functions would also be fine.

score 3 · Accepted Answer · answered Jan 15 '17 at 13:46

You need an explicit type annotation for f. GHC isn't able to infer the right type.

{-# LANGUAGE ImplicitParams, Rank2Types #-}

main = print (f z)

f :: ((?x :: Int, ?y :: Int) => a) -> a
f g =
  let 
    ?x = 42
    ?y = 5
  in
    g

z :: (?x :: Int) => Int
z = ?x

A function that sets an implicit parameter context

1 Answers1