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Make Let-Pattern desugaring less brittle #3268

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Merged
kritzcreek merged 1 commit into from
Mar 6, 2018
Merged

Make Let-Pattern desugaring less brittle #3268

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36 changes: 23 additions & 13 deletions src/Language/PureScript/Sugar/LetPattern.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,37 +6,47 @@ module Language.PureScript.Sugar.LetPattern (desugarLetPatternModule) where

import Prelude.Compat

import Data.List (groupBy, concatMap)
import Data.Function (on)

import Language.PureScript.AST
import Language.PureScript.Crash

-- |
-- Replace every @BoundValueDeclaration@ in @Let@ expressions with @Case@
-- | Replace every @BoundValueDeclaration@ in @Let@ expressions with @Case@
-- expressions.
--
desugarLetPatternModule :: Module -> Module
desugarLetPatternModule (Module ss coms mn ds exts) = Module ss coms mn (map desugarLetPattern ds) exts

-- |
-- Desugar a single let expression
--
-- | Desugar a single let expression
desugarLetPattern :: Declaration -> Declaration
desugarLetPattern decl =
let (f, _, _) = everywhereOnValues id replace id
in f decl
where
replace :: Expr -> Expr
replace (Let ds e) = go ds e
replace (Let ds e) = go (partitionDecls ds) e
replace other = other

go :: [Declaration]
go :: [Either [Declaration] (SourceAnn, Binder, Expr)]
-- ^ Declarations to desugar
-> Expr
-- ^ The original let-in result expression
-> Expr
go [] e = e
go (BoundValueDeclaration (pos, com) binder boundE : ds) e =
go (Right ((pos, com), binder, boundE) : ds) e =
PositionedValue pos com $ Case [boundE] [CaseAlternative [binder] [MkUnguarded $ go ds e]]
go (d:ds) e = append d $ go ds e
go (Left ds:dss) e = Let ds (go dss e)

partitionDecls :: [Declaration] -> [Either [Declaration] (SourceAnn, Binder, Expr)]
partitionDecls = concatMap f . groupBy ((==) `on` isBoundValueDeclaration)
where
f ds@(d:_)
| isBoundValueDeclaration d = map (Right . g) ds
f ds = [Left ds]

g (BoundValueDeclaration sa binder expr) = (sa, binder, expr)
g _ = internalError "partitionDecls: the impossible happened."

append :: Declaration -> Expr -> Expr
append d (Let ds e) = Let (d:ds) e
append d e = Let [d] e
isBoundValueDeclaration :: Declaration -> Bool
isBoundValueDeclaration BoundValueDeclaration{} = True
isBoundValueDeclaration _ = False