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type BaseTyCon = TyConNumber
               | TyConBool
               | TyConUnit
               | TyConList
               | TyConFun
               | TyConTuple
               | TyConUserDefined of string

type TyCon = TyCon of BaseTyCon * Kind

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type Kind = Star | KFun of Kind * Kind

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let rec kindStack numTyArgs =
    match numTyArgs with
    | 0 -> Star
    | n -> KFun (Star, kindStack (n - 1))

let tNumber = TConExpr (TyCon (TyConNumber, Star))
let tBool = TConExpr (TyCon (TyConBool, Star))
let tUnit = TConExpr (TyCon (TyConUnit, Star))

let tArrow = TConExpr (TyCon (TyConFun, KFun (Star, KFun (Star, Star))))

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let baseTyConString b =
    match b with
    | TyConNumber -> "Number"
    | TyConBool -> "Bool"
    | TyConUnit -> "()"
    | TyConUserDefined name -> name
    | _ -> sprintf "%A" b

let parens s = sprintf "(%s)" s

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let rec tysubst theta t =
    match t with
    | TVarExpr u ->
        match Map.tryFind u theta with
        | Some t' -> t'
        | None -> t
    | TApExpr (l, r) ->
        TApExpr (tysubst theta l, tysubst theta r)
    | _ ->
        t

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type ErrorMessage = Lazy<string>

type Constraint = Equal of TyExpr * TyExpr * ErrorMessage
                | And of Constraint * Constraint
                | Trivial

let (=~=) q1 q2 err = Equal (q1, q2, err)
let (&&&) c1 c2 = And (c1, c2)

let rec conjoinConstraints cc =

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exception TypeError of string

// Some useful extensions to F#'s map module
module Map =
    let keys m = m |> Map.toSeq |> Seq.map fst |> Set.ofSeq

    let fromKeys f keys =
        Seq.fold (fun accum k -> Map.add k (f k) accum) Map.empty keys

    let singleton key value =

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[<EntryPoint>]
let main argv =
    let solveAndCatch con =
        try
            printf "%A\n\n" (solve con)
        with
            | TypeError s ->
                printf "%s" s
    let t1 = tTuple [freshtyvar () |> TVarExpr; tNumber]
    let t2 = tTuple [tUnit; freshtyvar () |> TVarExpr]

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map
  [(TyVar ("t1",Star), TConExpr (TyCon (TyConUnit,Star)));
   (TyVar ("t2",Star), TConExpr (TyCon (TyConNumber,Star)))]

Type error: The types () and Number are not equal.

Clearly unit isn't the same type as a number

calebh

myconcat xs ys = xs ++ ys
greet = myconcat "Hello "