# HG changeset patch # User Adam Chlipala # Date 1224633380 14400 # Node ID 6a0e54400805251dfc6e9de25b4c264261ce2670 # Parent df4cbd90a26ed094fe76f5f5a57e84a24ddc5fef Sum prose diff -r df4cbd90a26e -r 6a0e54400805 demo/link.ur --- a/demo/link.ur Tue Oct 21 19:31:11 2008 -0400 +++ b/demo/link.ur Tue Oct 21 19:56:20 2008 -0400 @@ -3,5 +3,5 @@ fun main () = return - Go there + Go there diff -r df4cbd90a26e -r 6a0e54400805 demo/prose --- a/demo/prose Tue Oct 21 19:31:11 2008 -0400 +++ b/demo/prose Tue Oct 21 19:56:20 2008 -0400 @@ -71,3 +71,28 @@

Other functions demonstrate use of the dml function, for building a transaction from a SQL DML command. It is easy to insert antiquoted Ur code into queries and DML commands, and the type-checker catches mistakes in the types of the expressions that we insert.

+ +sum.urp + +

Metaprogramming is one of the most important facilities of Ur. This example shows how to write a function that is able to sum up the fields of records of integers, no matter which set of fields the particular record has.

+ +

Ur's support for analysis of types is based around extensible records, or row types. In the definition of the sum function, we see the type parameter fs assigned the kind {Unit}, which stands for records of types of kind Unit. The Unit kind has only one inhabitant, (). The kind Type is for "normal" types.

+ +

The unary $ operator is used to build a record Type from a {Type} (that is, the kind of records of types). The library function mapUT takes in a Type t and a {Unit} r, and it builds a {Type} as long as r, where every field is assigned value t.

+ +

Another library function foldUR is defined at the level of expressions, while mapUT is a type-level function. foldUR takes 6 arguments, some of them types and some values. Type arguments are distinguished by being written within brackets. The arguments to foldUR respectively tell us: + +

The type we will assign to each record field
The type of the final and all intermediate results of the fold, expressed as a function over the portion of the {Unit} that has been traversed so far
A function that updates the accumulator based on the current record field name, the rest of the input record type, the current record field value, and the current accumulator
The initial accumulator value
The input record type
The input record value

+ +An unusual part of the third argument is the syntax [t1 ~ t2] within a multi-argument fn. This syntax denotes a proof that row types t1 and t2 have no field names in common. The proof is not named, because it is applied automatically as needed. Indeed, the proof appears unused in this case, though it is actually needed to ensure the validity of some inferred types, as well as to unify with the type of foldUR.

+ +

The general syntax for constant row types is [Name1 = t1, ..., NameN = tN], and there is a shorthand version [Name1, ..., NameN] for records of Units.

+ +

With sum defined, it is easy to make some sample calls. The form of the code for main does not make it apparent, but the compiler must "reverse engineer" the appropriate {Unit} from the {Type} available from the context at each call to sum.

diff -r df4cbd90a26e -r 6a0e54400805 demo/sum.ur --- a/demo/sum.ur Tue Oct 21 19:31:11 2008 -0400 +++ b/demo/sum.ur Tue Oct 21 19:56:20 2008 -0400 @@ -4,6 +4,7 @@ 0 [fs] x fun main () = return + {[sum {}]}
{[sum {A = 0, B = 1}]}
{[sum {C = 2, D = 3, E = 4}]} diff -r df4cbd90a26e -r 6a0e54400805 src/elab_err.sig --- a/src/elab_err.sig Tue Oct 21 19:31:11 2008 -0400 +++ b/src/elab_err.sig Tue Oct 21 19:56:20 2008 -0400 @@ -49,7 +49,7 @@ | COccursCheckFailed of Elab.con * Elab.con | CIncompatible of Elab.con * Elab.con | CExplicitness of Elab.con * Elab.con - | CKindof of Elab.kind * Elab.con + | CKindof of Elab.kind * Elab.con * string | CRecordFailure of Elab.con * Elab.con val cunifyError : ElabEnv.env -> cunify_error -> unit diff -r df4cbd90a26e -r 6a0e54400805 src/elab_err.sml --- a/src/elab_err.sml Tue Oct 21 19:31:11 2008 -0400 +++ b/src/elab_err.sml Tue Oct 21 19:56:20 2008 -0400 @@ -109,7 +109,7 @@ | COccursCheckFailed of con * con | CIncompatible of con * con | CExplicitness of con * con - | CKindof of kind * con + | CKindof of kind * con * string | CRecordFailure of con * con fun cunifyError env err = @@ -131,8 +131,8 @@ eprefaces "Differing constructor function explicitness" [("Con 1", p_con env c1), ("Con 2", p_con env c2)] - | CKindof (k, c) => - eprefaces "Unexpected kind for kindof calculation" + | CKindof (k, c, expected) => + eprefaces ("Unexpected kind for kindof calculation (expecting " ^ expected ^ ")") [("Kind", p_kind k), ("Con", p_con env c)] | CRecordFailure (c1, c2) => diff -r df4cbd90a26e -r 6a0e54400805 src/elaborate.sml --- a/src/elaborate.sml Tue Oct 21 19:31:11 2008 -0400 +++ b/src/elaborate.sml Tue Oct 21 19:56:20 2008 -0400 @@ -481,7 +481,7 @@ (case hnormKind (kindof env c) of (L'.KArrow (_, k), _) => k | (L'.KError, _) => kerror - | k => raise CUnify' (CKindof (k, c))) + | k => raise CUnify' (CKindof (k, c, "arrow"))) | L'.CAbs (x, k, c) => (L'.KArrow (k, kindof (E.pushCRel env x k) c), loc) | L'.CDisjoint (_, _, _, c) => kindof env c @@ -497,7 +497,7 @@ | L'.CProj (c, n) => (case hnormKind (kindof env c) of (L'.KTuple ks, _) => List.nth (ks, n - 1) - | k => raise CUnify' (CKindof (k, c))) + | k => raise CUnify' (CKindof (k, c, "tuple"))) | L'.CError => kerror | L'.CUnif (_, k, _, _) => k @@ -546,7 +546,7 @@ case hnormKind (kindof env c) of (L'.KRecord k, _) => k | (L'.KError, _) => kerror - | k => raise CUnify' (CKindof (k, c)) + | k => raise CUnify' (CKindof (k, c, "record")) val k1 = rkindof c1 val k2 = rkindof c2 @@ -2318,7 +2318,7 @@ val env = if n1 = n2 then env else - E.pushCNamedAs env x n1 (L'.KType, loc) + E.pushCNamedAs env x n1 k' (SOME (L'.CNamed n2, loc)) in SOME (env, denv)