Add support for ML-style guards (#110)

which are patterns of the form `if e end`

Project.toml

@@ -1,6 +1,6 @@
 name = "Match"
 uuid = "7eb4fadd-790c-5f42-8a69-bfa0b872bfbf"
-version = "2.1.2"
+version = "2.2.0"
 authors = ["Neal Gafter <[email protected]>", "Kevin Squire <[email protected]>"]
 
 [deps]

README.md

@@ -56,6 +56,7 @@ for examples of this and other features.
 * `x && y` matches values which match both patterns `x` and `y`
 * `x, if condition end` matches only if `condition` is true (`condition` may use any variables that occur earlier in the pattern eg `(x, y, z where x + y > z)`)
 * `x where condition` An alternative form for `x, if condition end`
+* `if condition end` A boolean computed pattern. `x && if condition end` is another way of writing `x where condition`.
 * Anything else is treated as a constant and tested for equality
 * Expressions can be interpolated in as constants via standard interpolation syntax `\$(x)`.  Interpolations may use previously bound variables.
 

src/binding.jl

@@ -410,6 +410,14 @@ function bind_pattern!(
         pattern0, assigned = bind_pattern!(location, subpattern, input, binder, assigned)
         pattern1 = shred_where_clause(guard, false, location, binder, assigned)
         pattern = BoundAndPattern(location, source, BoundPattern[pattern0, pattern1])
+
+    elseif is_expr(source, :if, 2)
+        # if expr end
+        if !is_empty_block(source.args[2])
+            error("$(location.file):$(location.line): Unrecognized @match guard syntax: `$source`.")
+        end
+        guard = source.args[1]
+        pattern = shred_where_clause(guard, false, location, binder, assigned)
 
     elseif is_expr(source, :call) && source.args[1] == :(:) && length(source.args) in 3:4
         # A range pattern.  We depend on the Range API to make sense of it.

test/matchtests.jl

@@ -1,4 +1,4 @@
-@testset "tests from Match.jl" begin
+@testset "tests for Match.jl" begin
 
 @testset "Type matching" begin
     # Type matching
@@ -410,4 +410,39 @@ end
 
 end
 
+@testset "Test support for ML-style guards, which are patterns of the form `if expr end`" begin
+    t = true
+    f = false
+    @test @match 1 begin
+        if f end => false
+        if t end => true
+    end
+    # The original feature request was for ML-style guards of the form
+    #     pattern && if condition end => result
+    # as an alternative to guards of the forms
+    #     pattern, if condition end => result
+    #     pattern where condition => result
+    @test @match 1 begin
+        1 && if !t end => false
+        1 && if !f end => true
+    end
+end
+
+@testset "Test an error when ML-style guards are not used correctly" begin
+    let line = 0, file = Symbol(@__FILE__)
+        try
+            line = (@__LINE__) + 2
+            @eval @match Foo(1, 2) begin
+                if t; nothing; end => true
+            end
+            @test false
+        catch ex
+            @test ex isa LoadError
+            e = ex.error
+            @test e isa ErrorException
+            @test startswith(e.msg, "$file:$line: Unrecognized @match guard syntax:")
+        end
+    end
+end
+
 end