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'

Question

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 ?

Daniel · Accepted Answer · 2012-10-22 21:20:02Z

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}

And note that the correct name for double in F# is actually float.
@Jon Harrop: Thanks Jon, I was wondering about this as well, I did not start reading your book yet, I am still on "Beginning F#"
double and float are both aliases for System.Double, but yes, float is more common/idiomatic.

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