This construct causes code to be less generic than indicated by the type annotations. The type variable 'x has been constrained to be type y'

6
votes

This code will generate the following warning

module TimeSeries

open System

type TimedValue<'T> = { ts : DateTime; value: 'T}
type TimeSerie<'T> = TimedValue<'T> seq

let t : TimedValue<'double> = { ts = DateTime.Today; value=5}

Warning:

This construct causes code to be less generic than indicated by the type annotations. The type variable 'double has been constrained to be type 'int'.

I am quite new to F#, I think the 5 is interpreted as an int and somehow F# tells me that I asked a double but it will be an int.

When I tried replacing 5 with 5. this told me that it was still constrained by the float type.

Should I somehow cast it in double or just remove the declaration part : TimedValue<'double> and let F# deal with the types ?

1
typesf#

1 Answers

12
votes

Remove the apostrophe before double.

let t : TimedValue<double> = { ts = DateTime.Today; value=5.0}

A leading apostrophe is used to declare a type argument. So, you've declared a generic value but, by specifying value=5 you've constrained the type arg to be int. You could also use a wildcard in place of the type arg:

let t : TimedValue<_> = { ts = DateTime.Today; value=5.0}

or remove the type annotation completely:

let t = { ts = DateTime.Today; value=5.0}