lude

commit c5db2fca4fb3dd4a23cae34a8829a8b77fe8cbc0
parent 76d021fb8eb5ed9b65a8f1acdb7c9698641e5452
Author: Robert Russell <robert@rr3.xyz>
Date:   Thu, 15 Oct 2026 17:28:45 -0700

Haskell.Partial: init

Diffstat:
M
src/Lude/Haskell.hs
 | 
1
+
A
src/Lude/Haskell/Partial.hs
 | 
37
+++++++++++++++++++++++++++++++++++++

2 files changed, 38 insertions(+), 0 deletions(-)
diff --git a/src/Lude/Haskell.hs b/src/Lude/Haskell.hs
@@ -10,6 +10,7 @@ import Lude.Haskell.Constraint as X
 import Lude.Haskell.Core as X
 import Lude.Haskell.Functional as X
 import Lude.Haskell.Generic as X hiding (Rep, Rep1, from, from1, to, to1)
+import Lude.Haskell.Partial as X
 import Lude.Haskell.Tuple as X
 import Lude.Haskell.Type as X
 
diff --git a/src/Lude/Haskell/Partial.hs b/src/Lude/Haskell/Partial.hs
@@ -0,0 +1,37 @@
+module Lude.Haskell.Partial (
+    Partial,
+    unsafe_partial,
+) where
+
+import GHC.Exts qualified as GHC
+
+import Lude.Haskell.Constraint
+import Lude.Haskell.Core
+import Lude.Haskell.Type
+
+-- NOTE: We prevent writing an instance of Partial by only exporting a
+-- constraint synonym, which GHC disallows in the head of an instance
+-- declaration.
+
+-- NOTE: We need a dummy method to use `GHC.withDict`, hence `capability`.
+-- Keeping the constructor of that method private helps avoid users
+-- "accidentally" calling `GHC.withDict` on partial computations themselves.
+
+-- NOTE: This is inspired by PureScript's Partial constraint:
+-- https://pursuit.purescript.org/packages/purescript-partial
+
+--------------------------------------------------------------------------------
+
+type Capability :: Type
+data Capability = Unsafe_MkCapability
+
+type Partial_ :: Constraint
+class Partial_ where capability :: Capability
+
+type Partial :: Constraint
+type Partial = Partial_
+
+--------------------------------------------------------------------------------
+
+unsafe_partial :: forall {r :: Repr} (a :: Type# r). ((Partial) => a) -> a
+unsafe_partial = GHC.withDict Unsafe_MkCapability