purekell

Bidirectional Haskell/PureScript expression translator. Parses expressions from either language into a shared AST, then prints them back in the target language with correct syntax.

Built to complement purescript-bridge — while purescript-bridge generates PureScript types from Haskell types, purekell translates expressions (specifically typeclass instance method bodies).

Usage

import Purekell

-- Translate expressions between languages
hsToPs "uid x == uid y"           -- Right "x.uid == y.uid"
psToHs "x.uid == y.uid"           -- Right "uid x == uid y"

-- Parse and print instance method bodies
import Purekell.Instance

parseMethodBody "eq x y = x == y" -- Right [MethodEquation ...]
printMethodBody Haskell eqs       -- "eq x y = x == y"
printMethodBody PureScript eqs    -- "eq x y = x == y"

Supported syntax

Expressions: literals (int, float, char, string), variables, constructors, application, infix operators (symbolic and backtick), lambda, if/then/else, case/of, let/in, do notation, negation, tuples, lists, operator sections, where clauses, type annotations, record construction/update, qualified names.

Patterns: variable, constructor, literal, wildcard, tuple, list, cons, as-pattern, negated literal, record.

Types: constructors, variables, application, function arrows, qualified constructors.

Divergent syntax

Most syntax is shared between Haskell and PureScript. The printer handles these divergences:

Construct	Haskell	PureScript
Record access	field rec	rec.field
Tuples	(a, b, c)	Tuple a (Tuple b c)
Cons patterns	x : xs	Cons x xs
Record construction	Foo { bar = 1 }	Foo { bar: 1 }
Record patterns	Foo { bar = x }	Foo { bar: x }

Architecture

• Purekell.AST — Shared AST types (Expr, Pat, Type, Binding, etc.)
• Purekell.Parser — Megaparsec-based parser, parameterized for language-specific postfix (e.g. PS dot access)
• Purekell.Printer — Target-aware printer with correct parenthesization
• Purekell.Codec — Codec a pairs a parser with a printer; provides runParse, runPrint, roundtrip
• Purekell.Haskell / Purekell.PureScript — Language-specific codec instances
• Purekell.Instance — Parsing/printing of typeclass method equations (name pat1 pat2 = body)

Testing

cd purekell && stack test

331 tests covering parse, print, roundtrip (Haskell, PureScript, cross-language), and property-based roundtrip tests using QuickCheck with Arbitrary instances for all AST types.