CODEWITHSUNDEEP
f#
urig
urig

Reputation: 16841

F#: This expression was expected to have type DateTime but here has type unit

The below F# function does not compile

open System
let checkCreation time : DateTime = 
    if (time > DateTime.UtcNow.AddDays(-7.0)) then printfn "New"
    else printfn "Old"

checkCreation time

The error markers point to "New" and to "Old"

The compiler fails with the following error:

Script1.fsx(3,59): error FS0001: This expression was expected to have type

DateTime

but here has type

unit

Why does the compiler expect a DateTime when I'm just trying to print something via printfn?

Upvotes: 3

Views: 670

Answers (1)

Szer
Szer

Reputation: 3476

replace this

let checkCreation time: DateTime =

with this

let checkCreation (time: DateTime) =

First one has signature (DateTime -> DateTime) because you explicitly made this constraint to function output. Input has been inferred by compiler.

Second one has signature (DateTime -> unit). Input has been explicitly constrained, and output unit inferred

Added:

Full explicit signature should look like this

let checkCreation (time: DateTime) : unit = 
    ...

You could delete every explicit type constraint and make compiler do the work:

//because time argument compared to DateTime it is inferred to be DateTime
//No explicit constrain needed
let checkCreation time = //DateTime -> unit

    //because if expression is the last one, its output will be used as function output
    if (time > DateTime.UtcNow.AddDays(-7.0))
    //because then branch has unit output, function output will be inferred as unit
    then printfn "New" 
    //else branch output MUST match with then branch. Your code pass :)
    else printfn "Old"

Upvotes: 6

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