feat(python): support quotation derived patterns (#4928)

Author: dbrattli

src/fable-library-py/fable_library/quotation.py

@@ -653,3 +653,48 @@ def sub(e: Expr) -> Expr:
                 return e
 
     return sub(expr)
+
+
+# ===================================================================
+# DerivedPatterns
+# F# defines these on top of Patterns. AndAlso and OrElse recover the
+# shape `&&` and `||` desugar into; SpecificCall matches a call by the
+# identity of a template quotation rather than by compiled name.
+# ===================================================================
+
+
+def is_and_also(expr: Expr) -> tuple[Expr, Expr] | None:
+    """Match `a && b`, represented as `if a then b else false`."""
+    if isinstance(expr, ExprIfThenElse) and isinstance(expr.else_expr, ExprValue) and expr.else_expr.value is False:
+        return (expr.guard, expr.then_expr)
+    return None
+
+
+def is_or_else(expr: Expr) -> tuple[Expr, Expr] | None:
+    """Match `a || b`, represented as `if a then true else b`."""
+    if isinstance(expr, ExprIfThenElse) and isinstance(expr.then_expr, ExprValue) and expr.then_expr.value is True:
+        return (expr.guard, expr.else_expr)
+    return None
+
+
+def _template_call(expr: Expr) -> ExprCall | None:
+    """Find the call identifying a template quotation under its lambdas."""
+    current = expr
+    while isinstance(current, ExprLambda):
+        current = current.body
+    return current if isinstance(current, ExprCall) else None
+
+
+def is_specific_call(template: Expr, expr: Expr) -> tuple[Expr | None, FSharpList[str], FSharpList[Expr]] | None:
+    """Match a call by the method identity represented by a template quotation."""
+    wanted = _template_call(template)
+    if not isinstance(expr, ExprCall) or wanted is None:
+        return None
+    if expr.method != wanted.method or expr.declaring_type != wanted.declaring_type:
+        return None
+
+    instance = None if isinstance(expr.instance, ExprValue) and expr.instance.type == "novalue" else expr.instance
+    # The middle slot is F#'s generic-argument list. This runtime models types as
+    # names rather than System.Type, so it is always empty; callers match it with
+    # a wildcard.
+    return (instance, of_array(Array([])), of_array(expr.args))

tests/Python/TestQuotation.fs

@@ -4,6 +4,7 @@ open Fable.Tests.Util
 open Util.Testing
 open Microsoft.FSharp.Quotations
 open Microsoft.FSharp.Quotations.Patterns
+open Microsoft.FSharp.Quotations.DerivedPatterns
 open Microsoft.FSharp.Linq.RuntimeHelpers
 
 type QuotationTestUnion =
@@ -12,6 +13,26 @@ type QuotationTestUnion =
 
 let inline quotTestDouble x = x * 2
 
+// DerivedPatterns: AndAlso and OrElse recover the shape `&&` and `||` desugar
+// into, and SpecificCall matches a call by the identity of a template quotation
+// rather than by its compiled name. All three previously reported
+// "not supported by Fable".
+type private DpRec = { Country: string; Balance: float; Id: int }
+
+let rec private dpToSql (e: Expr) : string =
+    match e with
+    | Lambda(_, body) -> dpToSql body
+    | AndAlso(l, r) -> "(" + dpToSql l + " AND " + dpToSql r + ")"
+    | OrElse(l, r) -> "(" + dpToSql l + " OR " + dpToSql r + ")"
+    | SpecificCall <@ (=) @> (_, _, [ l; r ]) -> "(" + dpToSql l + " = " + dpToSql r + ")"
+    | SpecificCall <@ (>) @> (_, _, [ l; r ]) -> "(" + dpToSql l + " > " + dpToSql r + ")"
+    | SpecificCall <@ (<) @> (_, _, [ l; r ]) -> "(" + dpToSql l + " < " + dpToSql r + ")"
+    // Bound as a wildcard: PropertyGet exposes a PropertyInfo on Rust but the
+    // bare name on the other targets, and this test is about DerivedPatterns.
+    | PropertyGet _ -> "col"
+    | Value _ -> "?"
+    | _ -> "<unsupported>"
+
 [<Fact>]
 let ``test Simple integer value quotation`` () =
     let q = <@ 42 @>
@@ -283,3 +304,35 @@ let ``test Call on an instance whose value is null keeps a Some instance`` () =
 
     if not (hasSomeInstancePropertyGet q) then
         failwith "Expected a PropertyGet with a Some instance, even though its value is null"
+
+[<Fact>]
+let ``test DerivedPatterns AndAlso, OrElse and SpecificCall work`` () =
+    dpToSql <@ fun (c: DpRec) -> c.Country = "UK" @> |> equal "(col = ?)"
+
+    dpToSql <@ fun (c: DpRec) -> c.Country = "UK" && c.Balance > 100.0 @>
+    |> equal "((col = ?) AND (col > ?))"
+
+    dpToSql <@ fun (c: DpRec) -> c.Id < 5 || c.Balance > 1.0 @>
+    |> equal "((col < ?) OR (col > ?))"
+
+[<Fact>]
+let ``test a captured local is a Value, not a Var`` () =
+    let captured = "SE"
+
+    let describe (e: Expr) =
+        match e with
+        | Lambda(_, body) ->
+            match body with
+            | Value(v, _) -> "Value:" + unbox<string> v
+            | Var v -> "Var:" + v.Name
+            | _ -> "other"
+        | _ -> "not a lambda"
+
+    describe <@ fun (_: int) -> captured @> |> equal "Value:SE"
+    describe <@ fun (_: int) -> "SE" @> |> equal "Value:SE"
+
+    // Checks that a bound variable remains a Var, rather than being spliced in.
+    (match <@ fun (x: string) -> x @> with
+     | Lambda(_, Var _) -> "Var"
+     | _ -> "?")
+    |> equal "Var"