Printing And Formatting Outputs In F#

Amit Choudhary
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In this post we’ll discuss the very basic function that everyone uses in any language. When someone starts learning programming language the very first thing they want to do is printing/writing out on console.

The Print

F# has two basic functions; “printf” and “printfn” to print out the information on console.

// Theprintf/printfn functions are similar to the
// Console.Write/WriteLine functions in C#.

e.g.

printfn "Hello %s" "World"
printfn "it's %d" 2016

Formatting specifiers’ cheatsheet

The print functions in F# are statically type checked equivalent to C-style printing. So any argument will be type checked with format specifiers. Below is list of F# format specifiers.

%s for strings
%b for bools
%i or %d for signed ints%u for unsigned ints
%f for floats
%A for pretty-printing tuples, records and union types, BigInteger and all complex types
%O for other objects, using ToString()
%x and %X for lowercase and uppercase hex
%o for octal
%f for floats standard format
%e or %E for exponential format
%g or %G for the more compact of f and e.
%M for decimals

Padded formatting

%0i pads with zeros
%+i shows a plus sign
% i shows a blank in place of a plus sign

Date Formatting

There’s not built-in format specifier for date type. There are two options to format dates:

With override of ToString() method
Using a custom callback with ‘%a’ specifier which allows a callback function that take TextWriter as argument.

With option 1:

// function to format a date
let yymmdd1 (date:System.DateTime) = date.ToString("MM/dd/YYYY")
[<EntryPoint>]
let main argv =
printfn "using ToString = %s" (yymmdd1 System.DateTime.Now)
System.Console.ReadKey() |> ignore// To hold the console window
0 // return an integer exit code

With option 2:

// function to format a date onto a TextWriter
let yymmdd2 (tw:System.IO.TextWriter) (date:DateTime) = tw.Write("{0:yy.MM.dd}", date)

printfn "using a callback = %a" yymmdd2 System.DateTime.Now
Of course here option 1 is much easier but you can go with any of the options.

Note: Printing ‘%’ character will require escaping as it has special meaning.

printfn "unescaped: %" // error
printfn "escape: %%"

Read out more about F# print formatters here.