This page collects examples and sample DSLs implemented
in the typed final (``tagless-final'') style.

- A surprisingly simple implementation of staging
- lambda to SKI: a different view on the old translation
- Expressing sharing
- Effects Without Monads: Non-determinism -- embedding effectful DSLs
- Reconciling Abstraction with High Performance: A MetaOCaml approach
- Evaluators, Normalizers, Reducers: from denotational to operational semantics
- Ordinary and one-pass CPS transformation
- Type-directed partial evaluation
- Parameterizing expressions by the evaluation order: call-by-name, call-by-value, call-by-need
- Type-safe Formatted IO (printf and scanf) as an embedded DSL: the final view
- Tagless-Staged: a step toward MetaHaskell
- Type- and Scope-Safe Code Generation with Mutable Cells: A tagless-final OCaml embedding of a staged calculus with a rather complicated, MLF-like type system
- Math layout
- Linear and affine lambda-calculi
- Lambda: the ultimate syntax-semantics interface
- Lambek calculi
- Transformational Semantics (TS): Gradually Transforming Syntax to Semantics
- Mobile Code in C# via Finally Tagless Interpreters

- We demonstrate the ordinary and the administrative-redex--less call-by-value
Continuation Passing Style (CPS) transformations that assuredly produce
well-typed terms and are
*patently*total.Our goal here is not to evaluate, view or serialize a tagless-final term, but to transform it to another one. The result is a tagless-final term, which we can interpret in multiple ways: evaluate, view, or transform again. It is natural to require the result of transforming a well-typed term be well-typed. In the tagless-final approach that requirement is satisfied automatically: after all, only well-typed terms are expressible. We impose a more stringent requirement that a transformation be total. In particular, the fact that the transformation handles all possible cases of the source terms must be patently, syntactically clear. The complete coverage must be so clear that the metalanguage compiler should be able to see that, without the aid of extra tools.

Since the only thing we can do with tagless-final terms is to interpret them, the CPS transformer is written in the form of an interpreter. It interprets source terms yielding transformed terms, which can be interpreted in many ways. In particular, the terms can be interpreted by the CPS transformer again, yielding 2-CPS terms. CPS transformers are composable, as expected.

A particular complication of the CPS transform is that the type of the result is different from the type of the source term: the CPS transform translates not only terms but also types. Moreover, base types and the arrow type are translated in different ways. To express CPS, we need an interpreter that gives the meaning not only to terms but also to types. In particular, what the function types denote should be up to a particular interpreter. It turns out the existing tagless-final framework is up to that task: with the help of type families, we can after all define an instance of Symantics that interprets source types as CPS-transformed types.

The ordinary (Fischer or Plotkin) CPS transform introduces many administrative redices, which make the result too hard to read. Danvy and Filinski proposed a one-pass CPS transform, which relies on the metalanguage to get rid of the administrative redices. The one-pass CPS transform can be regarded as an example of the normalization-by-evaluation.

CPS.hs [10K]

Ordinary and one-pass CPS transforms and their compositionsTTFdBHO.hs [2K]

The simplest tagless-final transformer, from the De-Bruijn--based Symantics to the one based on the higher-order abstract syntaxOlivier Danvy and Andrzej Filinski. Representing Control: A Study of the CPS Transformation.

Mathematical Structures in Computer Science, 1992.

- We present the unexpectedly straightforward embedding of the staged
language that may be regarded as a simple version of MetaOCaml. In
fact, MetaOCaml quotations and splices can be translated to our
language in the most direct way. The language supports open
quotation (allowing free variables in fragments of the generated code)
and cross-stage persistence.
The staged language is embedded into OCaml in the tagless-final style, as the signature

`Code`

and its three implementations:`CodeReal`

(in terms of the ordinary OCaml),`CodeSting`

(performing code generation via strings), and`CodeCode`

(in terms of MetaOCaml). The`CodeReal`

implementation -- of the staged calculus is terms of the ordinary, unstaged OCaml -- embodies an*unstaging translation*. It is orders of magnitude simpler compared to the other such translations described in literature, requiring no fancy types or type-level operations. The staged language can be extended to support the polymorphic`let`

. **Version**- The current version is February 2017
**References**- let-insertion.pdf [225K]

Generating Code with Polymorphic let: A Ballad of Value Restriction, Copying and Sharing

Electronic Proceedings in Theoretical Computer Science (EPTCS) 241, 2017, pp. 1-22 (Post-Proceedings ML/OCaml 2015) DOI: 10.4204/EPTCS.241.1

Sec 3.1 of the paper describes a staged calculus in the form of code-generating combinators, and the translation of MetaOCaml quotations and splices into it. The section formulates the correctness properties of the translation.polylet.ml [17K]

The OCaml code for the embedding of the staged language into OCaml, and a few examplesClosing the Stage: From Staged Code to Typed Closures

The previous, immensely more complex unstaging translation{Tagless-Staged: a step toward MetaHaskell

Type- and Scope-Safe Code Generation with Mutable Cells

The tagless-final embedding, in Haskell and OCaml, of a staged calculus with a rather complicated, MLF-like type system. The type system ensures scope safety of the generated code even in the presence of effects.

- One may think that only those DSL can be embedded in Haskell whose
type system is a subset of that of Haskell. To counter that impression
we show how to faithfully embed typed
*linear*lambda calculus. Any bound variable must be referenced exactly once in abstraction's body. As before, only well-typed and well-formed terms are representable. Haskell as the metalanguage will statically reject as ill-typed the attempts to represent terms with a bound variable referenced several times -- or, as in the K combinator, never.We build on the embedding of the ordinary simply typed lambda calculus with De Bruijn indices described earlier. An object term of the type

`a`

was represented as a value of the type`repr h a`

, where the binary type constructor`repr`

is a member of the class`Symantics`

. Here`h`

stands for the type environment, assigning types to free variables (`hypotheses') of a term. Linear lambda calculus regards bound variables as representing resources; referencing a variable consumes the resource. We use the type environment for tracking the state of resources: available or consumed. The type environment becomes the*type state*. We follow the approach described in Variable (type)state `monad'.We represent linear lambda terms by Haskell values of the type

`repr hi ho a`

, where`hi`

stands for the variable state before evaluating the term and`ho`

represents the state after evaluating the term. To be more precise,`hi`

and`ho`

are the types of the variable states. We can determine the types and hence the state of the variables statically: As usual, the type checker does abstract interpretation. In our tagless-final encoding,`lam`

has the following typelam :: repr (F a,hi) (U,ho) b -> repr hi ho (a->b)

The expression`(lam e)`

has the type`repr hi ho (a->b)`

provided the body of abstraction,`e`

, has the type`repr (F a,hi) (U,ho) b`

. That is, in the environment extended with a term of the type`a`

, the body must produce the value of type`b`

. The body must consume the term at the top of the environment, changing the type of the first environment cell from`F a`

to`U`

(the type of the used variable).A trivial modification turns the embedding of the linear lambda-calculus into that of the affine lambda-calculus, which allows to ignore bound variables. K combinator becomes expressible.

LinearLC.hs [11K]

Commented code defining the typed linear lambda calculus and its two interpreters, to evaluate and to show linear lambda terms. Later we add general abstractions imposing no constraints on the use of bound variables.Jeff Polakow: Embedding a full linear Lambda calculus in Haskell

Proceedings of the ACM SIGPLAN Haskell Symposium, 2015, pp. 177-188.

Polakow's tagless-final linear lambda-calculus interpreter relies on higher-order abstract syntax, rather than De Bruijn indices of LinearLC.hs. He implements the full linear lambda calculus with additives and units.

- We present a technique for compiling lambda-calculus expressions into
SKI combinators. Unlike the well-known bracket abstraction based on
(syntactic) term re-writing, our algorithm relies on a specially
chosen,
*compositional*semantic model of generally open lambda terms. The meaning of a closed lambda term is the corresponding SKI combination. For simply-typed as well as unityped terms, the meaning derivation mirrors the typing derivation. One may also view the algorithm as an algebra, or a non-standard evaluator for lambda-terms (i.e., denotational semantics).The algorithm is implemented as a tagless-final compiler for (uni)typed lambda-calculus embedded as a DSL into OCaml. Its type preservation is clear even to OCaml. The correctness of both the algorithm and of its implementation becomes clear.

Our algorithm is easily amenable to optimizations. In particular, its output and the running time can both be made linear in the size (i.e., the number of all constructors) of the input De Bruijn-indexed term.

**Version**- The current version is May 2018
**References**- ski.pdf [354K]

The paper presented on May 9, 2018 at FLOPS 2018 (Nagoya, Japan) and published in Springer's LNCS 10818, pp. 33-50, 2018

doi:10.1007/978-3-319-90686-7_3skconv.ml [34K]

The complete code accompanying the paper

The code presents the basic SKI translation and all optimizations described in the paper, along with several interpreters.

- Olivier Danvy's original POPL96 paper on type-directed partial
evaluation used an untyped target language, represented as an
algebraic data type. Type preservation was not apparent and had
to be proved. In our presentation, the result of reification is a
*typed*expression, in the tagless-final form. Type preservation of reification is now syntactically apparent and is verified by the Haskell type-checker. In the tagless-final presentation, reification and reflection seem particularly symmetric, elegant and insightful.TDPE.hs [6K]

Tagless-final presentation of type-directed partial evaluationToTDPE.hs [<1K]

The imported module with sample functions to reify. Compiling this module makes for a nicer example.<http://www.brics.dk/~danvy/tdpe-ln.pdf>

Olivier Danvy: Lecture notes on type-directed partial evaluation. The lecture notes are based on his POPL96 paper.

- We demonstrate a tagless-final embedding of the simply-typed
lambda-calculus with integers and constants, and its uniform
interpretations with three different evaluation strategies:
call-by-name, call-by-value and call-by-need. The three interpreters
share almost all the code, differing only in the interpretation of
object-language abstractions (
`lam`

-forms). The semantics of abstraction is indeed what sets the three evaluation orders apart. The examples also implement tracing of the interpreter actions, to make clear which expressions are evaluated, in which sequence and how many times. The interpreters are assuredly and patently type-preserving: the host language -- OCaml and Haskell -- guarantee that. We thus obtain a higher-order embedded domain-specific language with the selectable evaluation order.In Haskell, we define the DSL as a type class

`Symantics`

. The type class declaration specifies the syntax and its instances define the semantics of the language. Our DSL is typed. We use the Haskell type constant`Int`

to also denote the DSL type of integers (although we could have used a different name). DSL function types are represented in Haskell by the type synonym`Arr repr a b`

:class Symantics repr where int :: Int -> repr Int -- integer literals add :: repr Int -> repr Int -> repr Int -- addition sub :: repr Int -> repr Int -> repr Int -- and subtraction app :: repr (Arr repr a b) -> repr a -> repr b -- application type Arr repr a b = repr a -> repr b

This somewhat unusual representation of DSL functions permits the uniform implementation of evaluation strategies. The class`Semantics`

specifies the part of our calculus that is interpreted the same way in all evaluation strategies. Only DSL abstractions are interpreted differently; therefore, we make them a distinct DSL `feature', defined by its own type class:class SymLam repr where lam :: (repr a -> repr b) -> repr (Arr repr a b)

After introducing the convenient `macro'`let_`

(which could have been called `bind') we write a sample object language term as follows:let_ :: (Symantics repr, SymLam repr) => repr a -> (repr a -> repr b) -> repr b let_ x y = (lam y) `app` x -- The representation of the lambda-calculus term -- (\z x -> let y = x + x in y + y) (100 - 10) (5 + 5) t2 :: (Symantics repr, SymLam repr) => repr Int t2 = (lam $ \z -> lam $ \x -> let_ (x `add` x) $ \y -> y `add` y) `app` (int 100 `sub` int 10) `app` (int 5 `add` int 5)

The Haskell expression`t2`

represents the lambda-calculus term`(\z x -> let y = x + x in y + y) (100 - 10) (5 + 5)`

; the variable`z`

does not occur in the body of the abstraction whereas`x`

occurs twice.We are ready to write interpreters for our DSLs. We will interpret DSL expressions of the type

`a`

as Haskell values of the type`m a`

where`m`

is some`MonadIO`

.`IO`

is needed to print the trace of interpreter actions. The representation is common to the three evaluation strategies. To distinguish them (when interpreting`lam`

later on) we add a phantom type parameter, the label`l`

. It will take the values`Name`

,`Value`

and`Lazy`

; more strategies may be added later on.newtype S l m a = S { unS :: m a } deriving (Monad, Applicative, Functor, MonadIO)

Here is the code common to all interpreters regardless of the evaluation strategy. The reasons to parameterize the interpreter over`MonadIO`

is to print out the evaluation trace, so that we can see the difference among the evaluation strategies in the number of performed additions and subtractions.instance MonadIO m => Symantics (S l m) where int = return add x y = do a <- x b <- y liftIO $ putStrLn "Adding" return (a + b) sub x y = do a <- x b <- y liftIO $ putStrLn "Subtracting" return (a - b) app x y = x >>= ($ y)

Call-by-name is defined by the following interpretation of DSL abstractions:data Name -- The label instance Monad m => SymLam (S Name m) where lam f = return f

The abstraction, when applied, will receive an unevaluated argument expression -- of the type`repr a`

, or`S Name m a`

-- using (substituting) it as it is.We evaluate the sample term under call-by-name

runName :: S Name m a -> m a runName x = unS x t2SN = runName t2 >>= print

obtaining the result 40 and observing from the trace that subtraction was not performed: the value of`int 100 `sub` int 10`

was not needed to compute the result of`t2`

. On the other hand, the sub-expression`int 5 `add` int 5`

was evaluated four times.Call-by-value differs from call-by-name only in the interpretation of abstractions:

data Value instance Monad m => SymLam (S Value m) where -- lam f = return (\x -> (f . return) =<< x) -- or, in the pointless notation, as below: lam f = return (f . return =<<)

As in the call-by-name evaluator, the lambda-abstraction receives an unevaluated argument expression -- evaluating it before passing its result to the abstraction body`f`

. This is literally the definition of call-by-value. The very same sample term`t2`

can be interpreted differently:runValue :: S Value m a -> m a runValue x = unS x t2SV = runValue t2 >>= print

giving in the end the same result 40. Although the result of the subtraction`int 100 `sub` int 10`

in`t2`

was not needed, the trace shows it performed. On the other hand, the argument sub-expression`int 5 `add` int 5`

was evaluated only once. In call-by-value, arguments are evaluated exactly once.The call-by-need evaluator differs from the others again in one line, the interpretation of abstractions:

data Lazy instance MonadIO m => SymLam (S Lazy m) where -- lam f = return (\x -> f =<< share x) -- Or, in the pointless notation lam f = return ((f =<<) . share)

The evaluation of the body of the abstraction always starts by lazy sharing the argument expression. (The implementation of`share`

is straightforward; see the code for details.) Again, this is the definition of call-by-need. We run the very same term`t2`

with the new evaluator, obtaining the same result 40 and observing from the execution trace that subtraction was not evaluated (because it was not needed) but the needed argument expression`int 5 `add` int 5`

was evaluated once. In call by need, application arguments are evaluated*at most*once. **Version**- The current version is 1.2, March 17, 2010
**References**- CBAny.hs [7K]

The complete code with examples and testscall_by_any.ml [6K]

The OCaml version of the code, using modules and functorsCB.hs [7K]

The older version of the code

It was originally posted as ``CBN, CBV, Lazy in the same final tagless framework'' on the Haskell-Cafe mailing list on Thu, 8 Oct 2009 00:54:14 -0700 (PDT).CB98.hs [7K]

The same code in Haskell98, proving that the tagless final approach indeed requires fewer fancy type system features. The fancy features like the type families, used in CB.hs, add convenience and gloss. One can do without them however. The code can easily be re-written in OCaml, which has no typeclasses, type families and other bleeding-edge features.Parameterized syntax: interpreters without run-time interpretive overhead

Evaluating the same Scheme source language expression using call-by-value, call-by-reference, and call-by-name evaluation strategies.

- Jacques Carette has posed a problem of designing an embedded DSL for
Math layout, with subscripts, superscripts,
tables, fractions, and other standard features.
The language has to support an
*extensible*set of glyphs, such as Greek letters and special characters. It should be possible to add more glyphs later, similar to `using'`stmaryrd`

, etc. in LaTeX. Adding a new glyph should not affect the existing documents in any way. Another requirement is an extensible set of renderers (formatters), such as plain text, LaTeX, HTML+MathML, etc. One should be able to add a new formatter later, without having to re-write or even recompile the existing documents. A formatter does not have to support all layout features or glyphs. However, an attempt to render a document with an unsuitable formatter should be flagged as a type error.The tagless-final approach seems to easily fulfill all the requirements, as the enclosed code demonstrates.

**Version**- The current version is May 2016
**References**- MathLayout.hs [5K]

Haskell code outlining the embedding of the layout language, with LaTeX and HTML backends

- Lambek calculus is a resource-sensitive calculus introduced by Lambek
in 1958, almost three decades before linear logic. Like linear logic,
Lambek calculus does not have the weakening rule. In fact, in the
non-associative Lambek calculus NL, the antecedent of a sequent is a
tree and there are
*no*structural rules at all. Lambek calculus is hence the purest, simplest, and the earliest substructural logic. Adding the associativity and commutativity rules (that is, treating the antecedent as a sequence rather than a tree, and allowing exchange) turns Lambek calculus into a fragment of the Multiplicative Linear Logic (MILL).The most noticeable difference of Lambek calculus from the conventional or linear lambda-calculus is its

*directional*implications and abstractions. There are two function types typically written as`B / A`

and`A \ B`

, called left/right slash-types rather than arrow types. The function of the type`B / A`

accepts the argument of the type`A`

on the right;`A \ B`

accepts the`A`

argument on the left. There are hence two rules for eliminating implications and, correspondingly, two rules for introducing them, which bring in the power of hypothetical reasoning. Although the NL calculus per se has no structural rules, various NL theories add so-called structural postulates: the ways to rearrange the antecedent structure in particular limited ways. All these features set Lambek calculi even farther apart from the Haskell type system. And yet it can be embedded in Haskell, in the tagless-final approach. All and only valid derivations are represented by Haskell values of a particular`repr`

type. One tagless-final interpreter prints the ``yield'', the constants used in the derivation. In linguistic applications the yield spells out the sentence whose parse is represented by the derivation. Other interpreters transform the derivation to a logical formula that stands for the meaning of the sentence.A so-called symmetric, Lambek-Grishin calculus has, in addition to directional implications, directional co-implications and interesting symmetric structural rules for moving between the antecedent and the consequent structures. It too can be represented in the tagless-final style. The semantic interpretation builds the meaning formula in the continuation-passing style.

**References**- Michael Moortgat: Typelogical Grammar

The Stanford Encyclopedia of Philosophy (Spring 2014 Edition), Edward N. Zalta (ed.)

<http://plato.stanford.edu/entries/typelogical-grammar/>HOCCG.hs [28K]

Explanation of the code

Non-associative Lambek calculus NL with the non-standard semantic interpretation. Applications to quantification, non-canonical coordination and gapping.LG.hs [10K]

Tagless-final embedding of the Lambek-Grishin symmetric calculus and its 1-CPS translation

Our starting point is the regular CBV CPS translation for lambda-LG, described on p. 697 of the paper by Michael Moortgat ``Symmetric Categorial Grammar''. J. Philos. Logic, 2009. The original translation (eq (20) of the paper) yields many administrative beta redices. The present translation uses lightweight staging to remove such redices in the process.

- Sandro Magi shows that different interpretations of the same DSL term may not only involve different run-time systems but also occur on different hosts.
**Version**- The current version is June 23, 2009
**References**- <http://higherlogics.blogspot.com/2009/06/mobile-code-in-c-via-finally-tagless.html>

The article by Sandro Magi