F# printfn error

I tried to write out a message to the command line using ‘printfn’. Here is my code:

	let message = "Hello world!"
	printfn message // throws a compiler error

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Unfortunately this code doesn’t compile. Here is the compiler error I got:

The type 'string' is not compatible with the type 'Printf.TextWriterFormat<'a>'

How hard can it be to write a simple string message?

The problem becomes obvious when we look at the signature of the printfn function:

printfn : TextWriterFormat<'T> –> 'T

The function didn’t expect a string but a TextWriterFormat<'T>. The compiler has no clue how to convert our string variable to this type.

To fix this you should specify a format specification(%s in this case) and your variable:

	let message = "Hello world!"
	printfn "%s" message

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