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#light
open System

let rec fib a = 
    match a with
    | 1 -> 1
    | 2 -> 1
    | a -> fib(a-1) + fib(a-2)

print_endline(fib 5 |> any_to_string) 

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Building Solution beginner

Building Project: beginner Configuration: Debug

Performing main compilation...

System.NullReferenceException: Object reference not set to an instance of an object

  at Microsoft.FSharp.Compiler.Patcompile.compactify$rest_TLR_1@256@256 (Microsoft.FSharp.Option curr$env0@256, Microsoft.FSharp.List edges$env1@256) [0x00000] 

  at Microsoft.FSharp.Compiler.Patcompile.build_switch@193 (Microsoft.FSharp.Compiler.tcGlobals g, Microsoft.FSharp.FastFunc get_subexpr, System.Object expr, Microsoft.FSharp.List edges, Microsoft.FSharp.Option dflt, Int64 m) [0x00000] 

  at Microsoft.FSharp.Compiler.Patcompile.compile$rest_TLR_1@421@421 (Microsoft.FSharp.Compiler.tcGlobals g$env0@421, Int64 exprm$env1@421, Microsoft.FSharp.List topgtvs$env2@421, Microsoft.FSharp.List clauses$env3@421, Int32 nclauses$env4@421, Microsoft.FSharp.Lazy incompleteMatchClause$env5@421, Microsoft.FSharp.FastFunc discrim_of_switchpat$env6@421, Microsoft.FSharp.FastFunc get_subexpr$env7@421, Microsoft.FSharp.List frontiers$env8@421, Microsoft.FSharp.Compiler.subexpr_of_expr expr$env9@421, Microsoft.FSharp.List path$env10@421) [0x00000] 

  at Microsoft.FSharp.Compiler.Patcompile.compile$rest_TLR_1@363@363 (Microsoft.FSharp.Compiler.tcGlobals g$env0@363, Int64 exprm$env1@363, Microsoft.FSharp.List topgtvs$env2@363, Microsoft.FSharp.List clauses$env3@363, Int32 nclauses$env4@363, Microsoft.FSharp.Lazy incompleteMatchClause$env5@363, Microsoft.FSharp.FastFunc discrim_of_switchpat$env6@363, Microsoft.FSharp.FastFunc get_subexpr$env7@363, Microsoft.FSharp.List frontiers$env8@363) [0x00000] 

  at Microsoft.FSharp.Compiler.Patcompile.patcompile (Microsoft.FSharp.Compiler.tcGlobals g, Int64 exprm, Int64 matchm, Microsoft.FSharp.Compiler.onfail onfail, Microsoft.FSharp.Compiler.osgn topv, Microsoft.FSharp.List topgtvs, Microsoft.FSharp.List clauses, Microsoft.FSharp.Compiler.typ ty) [0x00000] 

  at Microsoft.FSharp.Compiler.Tc.patcompile_match@5850 (Microsoft.FSharp.Compiler.cenv cenv, Int64 exprm, Int64 matchm, Microsoft.FSharp.Compiler.onfail onfail, Microsoft.FSharp.Compiler.osgn v, Microsoft.FSharp.List gtps, Microsoft.FSharp.List clauses, Microsoft.FSharp.Compiler.typ rty) [0x00000] 

  at Microsoft.FSharp.Compiler.Tc.tc_matches@5838 (Int64 exprm, Int64 matchm, Microsoft.FSharp.Compiler.onfail onfail, Microsoft.FSharp.Compiler.cenv cenv, Microsoft.FSharp.Compiler.typ dty, Microsoft.FSharp.Compiler.typ rty, Microsoft.FSharp.Compiler.tenv env, Microsoft.AbstractIL.Internal.t tpenv, Microsoft.FSharp.List matches) [0x00000] 

  at Microsoft.FSharp.Compiler.Tc.tc_expr_undelayed@4682 (Microsoft.FSharp.Compiler.cenv cenv, Microsoft.FSharp.Compiler.typ ty, Microsoft.FSharp.Compiler.tenv env, Microsoft.AbstractIL.Internal.t tpenv, Microsoft.FSharp.Compiler.expr expr) [0x00000] 

  at Microsoft.FSharp.Compiler.Tc.tc_expr_then@4567 (Microsoft.FSharp.Compiler.cenv cenv, Microsoft.FSharp.Compiler.typ ty, Microsoft.FSharp.Compiler.tenv env, Microsoft.AbstractIL.Internal.t tpenv, Microsoft.FSharp.Compiler.expr expr, Microsoft.FSharp.List delayed) [0x00000] 

  at Microsoft.FSharp.Compiler.Tc.tc_expr@4499 (Microsoft.FSharp.Compiler.cenv cenv, Microsoft.FSharp.Compiler.typ ty, Microsoft.FSharp.Compiler.tenv env, Microsoft.AbstractIL.Internal.t tpenv, Microsoft.FSharp.Compiler.expr expr) [0x00000]