Question

In the following code, the where value= clause returns a partially applied function.

How can I ensure it gets fully applied?

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future_value :: Float -> Float -> Float -> Float
future_value present interest periods =
  (present * (( 1 + interest) ** periods))

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-- Given an initial amount
-- Given a yearly fee, as well as a yearly interest...
-- Calculates return over a number of years, given a yearly fee.
return_over_time :: Float -> Float -> Float -> Float -> Float
return_over_time present interest num_years fee =
  if num_years == 1 then (future_value  present interest 1.0) - fee
  else future_value value
       where value =  return_over_time present interest (num_years - 1) fee

Answer 1

The issue isn’t with value but with future_value. future_value takes three arguments but you only give it one (value), so future_value value has the type Float -> Float - > Float. value, on the other hand, is just a Float because return_over_time is fully applied in the where clause.

You can make sure future_value is fully applied by passing in two more floats for interest and periods.

Incidentally, is there any particular reason you are using Float instead of Double?