Another problem done

exercism/fsharp/grains/Grains.fs

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+module Grains
+
+let rec safeSqaure n: uint64 =
+    if n = 1 then uint64 (1) else uint64 (2) * safeSqaure (n - 1)
+
+let square (n: int): Result<uint64, string> =
+    if n < 1 || n > 64 then Error "square must be between 1 and 64" else Ok(safeSqaure n)
+
+let total: Result<uint64, string> =
+    Ok
+        (List.sumBy (fun x ->
+            let s = square x
+            match s with
+            | Ok (n) -> n
+            | _ -> uint64 (0)) [ 1 .. 64 ])

exercism/fsharp/grains/Grains.fsproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>netcoreapp3.0</TargetFramework>
+
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="Grains.fs" />
+    <Compile Include="GrainsTests.fs" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.6.1" />
+    <PackageReference Include="xunit" Version="2.4.1" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.2" />
+    <PackageReference Include="FsUnit.xUnit" Version="3.9.0" />
+  </ItemGroup>
+
+</Project>

exercism/fsharp/grains/GrainsTests.fs

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+// This file was auto-generated based on version 1.2.0 of the canonical data.
+
+module GrainsTests
+
+open FsUnit.Xunit
+open Xunit
+
+open Grains
+
+[<Fact>]
+let ``1`` () =
+    let expected: Result<uint64,string> = Ok 1UL
+    square 1 |> should equal expected
+
+[<Fact>]
+let ``2`` () =
+    let expected: Result<uint64,string> = Ok 2UL
+    square 2 |> should equal expected
+
+[<Fact>]
+let ``3`` () =
+    let expected: Result<uint64,string> = Ok 4UL
+    square 3 |> should equal expected
+
+[<Fact>]
+let ``4`` () =
+    let expected: Result<uint64,string> = Ok 8UL
+    square 4 |> should equal expected
+
+[<Fact>]
+let ``16`` () =
+    let expected: Result<uint64,string> = Ok 32768UL
+    square 16 |> should equal expected
+
+[<Fact>]
+let ``32`` () =
+    let expected: Result<uint64,string> = Ok 2147483648UL
+    square 32 |> should equal expected
+
+[<Fact>]
+let ``64`` () =
+    let expected: Result<uint64,string> = Ok 9223372036854775808UL
+    square 64 |> should equal expected
+
+[<Fact>]
+let ``Square 0 raises an exception`` () =
+    let expected: Result<uint64,string> = Error "square must be between 1 and 64"
+    square 0 |> should equal expected
+
+[<Fact>]
+let ``Negative square raises an exception`` () =
+    let expected: Result<uint64,string> = Error "square must be between 1 and 64"
+    square -1 |> should equal expected
+
+[<Fact>]
+let ``Square greater than 64 raises an exception`` () =
+    let expected: Result<uint64,string> = Error "square must be between 1 and 64"
+    square 65 |> should equal expected
+
+[<Fact>]
+let ``Returns the total number of grains on the board`` () =
+    let expected: Result<uint64,string> = Ok 18446744073709551615UL
+    total |> should equal expected
+

exercism/fsharp/grains/README.md

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+# Grains
+
+Calculate the number of grains of wheat on a chessboard given that the number
+on each square doubles.
+
+There once was a wise servant who saved the life of a prince. The king
+promised to pay whatever the servant could dream up. Knowing that the
+king loved chess, the servant told the king he would like to have grains
+of wheat. One grain on the first square of a chess board, with the number
+of grains doubling on each successive square.
+
+There are 64 squares on a chessboard (where square 1 has one grain, square 2 has two grains, and so on).
+
+Write code that shows:
+- how many grains were on a given square, and
+- the total number of grains on the chessboard
+
+## For bonus points
+
+Did you get the tests passing and the code clean? If you want to, these
+are some additional things you could try:
+
+- Optimize for speed.
+- Optimize for readability.
+
+Then please share your thoughts in a comment on the submission. Did this
+experiment make the code better? Worse? Did you learn anything from it?
+
+## Hints
+For this exercise the following F# features come in handy:
+- [BigInt](https://msdn.microsoft.com/en-us/visualfsharpdocs/conceptual/numerics.biginteger-structure-%5Bfsharp%5D)
+- [Seq.sumBy](https://msdn.microsoft.com/en-us/visualfsharpdocs/conceptual/seq.sumby%5B't,%5Eu%5D-function-%5Bfsharp%5D) is a condensed format to apply a function to a sequence and then sum the results
+
+## Running the tests
+
+To run the tests, run the command `dotnet test` from within the exercise directory.
+
+## Autoformatting the code
+
+F# source code can be formatted with the [Fantomas](https://github.com/fsprojects/fantomas) tool.
+
+After installing it with `dotnet tool restore`, run `dotnet fantomas .` to format code within the current directory.
+
+## Further information
+
+For more detailed information about the F# track, including how to get help if
+you're having trouble, please visit the exercism.io [F# language page](http://exercism.io/languages/fsharp/resources).
+
+## Source
+
+JavaRanch Cattle Drive, exercise 6 [http://www.javaranch.com/grains.jsp](http://www.javaranch.com/grains.jsp)
+