Language.Haskell.TH.Alpha

Compare TH expressions (or clauses, patterns, etc.) for alpha equivalence. That is, compare for equality modulo the renaming of bound variables.

>>> areExpAEq [| \x -> x |] [| \y -> y |]
True

This can be useful when for instance testing libraries that use Template Haskell: usually correctness is only defined up to alpha equivalence.

For most cases, areExpAEq is the only function you'll need. The AlphaEq class is only exported to make it easier to expand alpha-equality comparison to other instances, or to be used (in combination with the package __th-desugar__) for alpha equality comparisons of non-expression types (e.g. TyVarBndr).

N.B.: This package doesn't yet handle type annotations correctly!

Documentation

areExpAEq

Arguments

:: Quasi m

=> ExpQ

Quoted expression

-> ExpQ

Quoted expression

-> m Bool

Convenience function that uses runQ on expEqual.

>>> areExpAEq [| let x = 5 in x |] [| let y = 5 in y |]
True

expEqual :: Quasi m => Exp -> Exp -> m Bool

(@=) :: (Monad m, AlphaEq a m) => a -> a -> m Bool

Compares its arguments for alpha equivalence. The default implementation uses Lookup for its LookupTbl, but more efficient datatypes can be used.

class AlphaEq a m | a -> m where

The main Alpha Equivalence class. @= is by default defined in terms of lkEq. lkEq is exposed for composability: it is easy to recursively build AlphaEq instances from other AlphaEq instances by delegating the lookup update to the subinstances.

Methods

lkEq :: a -> a -> LookupSTM m ()

Given a variable binding lookup compares arguments for alpha equivalence, setting the state to Just of updated lookup in case of equivalence, Nothing otherwise.

Instances

AlphaEq Exp Q

AlphaEq DExp Identity

AlphaEq DPat Identity

AlphaEq DType Identity

AlphaEq DKind Identity

AlphaEq DPred Identity

AlphaEq DMatch Identity

AlphaEq DClause Identity

AlphaEq DLetDec Identity