Curried versions of endswith and startswith

NEWS.md

@@ -61,6 +61,7 @@ New library functions
   expressions before they are evaluated ([#34595]).
 * New function `bitreverse` for reversing the order of bits in a fixed-width integer ([#34791]).
 * New function `bitrotate(x, k)` for rotating the bits in a fixed-width integer ([#33937]).
+* One argument methods `startswith(x)` and `endswith(x)` have been added, returning partially-applied versions of the functions, similar to existing methods like `isequal(x)` ([#33193]).
 
 New library features
 --------------------

base/strings/util.jl

@@ -70,6 +70,36 @@ function endswith(a::Union{String, SubString{String}},
     end
 end
 
+"""
+    endswith(suffix)
+
+Create a function that checks whether its argument ends with `suffix`, i.e.
+a function equivalent to `y -> endswith(y, suffix)`.
+
+The returned function is of type `Base.Fix2{typeof(endswith)}`, which can be
+used to implement specialized methods.
+
+!!! compat "Julia 1.5"
+    The single argument `endswith(suffix)` requires at least Julia 1.5.
+
+"""
+endswith(s) = Base.Fix2(endswith, s)
+
+"""
+    startswith(prefix)
+
+Create a function that checks whether its argument starts with `prefix`, i.e.
+a function equivalent to `y -> startswith(y, prefix)`.
+
+The returned function is of type `Base.Fix2{typeof(startswith)}`, which can be
+used to implement specialized methods.
+
+!!! compat "Julia 1.5"
+    The single argument `startswith(prefix)` requires at least Julia 1.5.
+
+"""
+startswith(s) = Base.Fix2(startswith, s)
+
 """
     chop(s::AbstractString; head::Integer = 0, tail::Integer = 1)
 

test/strings/basic.jl

@@ -51,71 +51,119 @@ end
 
 @testset "{starts,ends}with" begin
     @test startswith("abcd", 'a')
+    @test startswith('a')("abcd")
     @test startswith("abcd", "a")
+    @test startswith("a")("abcd")
     @test startswith("abcd", "ab")
+    @test startswith("ab")("abcd")
     @test !startswith("ab", "abcd")
+    @test !startswith("abcd")("ab")
     @test !startswith("abcd", "bc")
+    @test !startswith("bc")("abcd")
     @test endswith("abcd", 'd')
+    @test endswith('d')("abcd")
     @test endswith("abcd", "d")
+    @test endswith("d")("abcd")
     @test endswith("abcd", "cd")
+    @test endswith("cd")("abcd")
     @test !endswith("abcd", "dc")
+    @test !endswith("dc")("abcd")
     @test !endswith("cd", "abcd")
+    @test !endswith("abcd")("cd")
     @test startswith("ab\0cd", "ab\0c")
+    @test startswith("ab\0c")("ab\0cd")
     @test !startswith("ab\0cd", "ab\0d")
+    @test !startswith("ab\0d")("ab\0cd")
     x = "∀"
     y = String(codeunits(x)[1:2])
     z = String(codeunits(x)[1:1])
     @test !startswith(x, y)
+    @test !startswith(y)(x)
     @test !startswith(x, z)
+    @test !startswith(z)(x)
     @test !startswith(y, z)
+    @test !startswith(z)(y)
     @test startswith(x, x)
+    @test startswith(x)(x)
     @test startswith(y, y)
+    @test startswith(y)(y)
     @test startswith(z, z)
+    @test startswith(z)(z)
     x = SubString(x)
     y = SubString(y)
     z = SubString(z)
     @test !startswith(x, y)
+    @test !startswith(y)(x)
     @test !startswith(x, z)
+    @test !startswith(z)(x)
     @test !startswith(y, z)
+    @test !startswith(z)(y)
     @test startswith(x, x)
+    @test startswith(x)(x)
     @test startswith(y, y)
+    @test startswith(y)(y)
     @test startswith(z, z)
+    @test startswith(z)(z)
     x = "x∀y"
     y = SubString("x\xe2\x88y", 1, 2)
     z = SubString("x\xe2y", 1, 2)
     @test !startswith(x, y)
+    @test !startswith(y)(x)
     @test !startswith(x, z)
+    @test !startswith(z)(x)
     @test !startswith(y, z)
+    @test !startswith(z)(y)
     @test startswith(x, x)
+    @test startswith(x)(x)
     @test startswith(y, y)
+    @test startswith(y)(y)
     @test startswith(z, z)
+    @test startswith(z)(z)
     x = "∀"
     y = String(codeunits(x)[2:3])
     z = String(codeunits(x)[3:3])
     @test !endswith(x, y)
+    @test !endswith(y)(x)
     @test !endswith(x, z)
+    @test !endswith(z)(x)
     @test endswith(y, z)
+    @test endswith(z)(y)
     @test endswith(x, x)
+    @test endswith(x)(x)
     @test endswith(y, y)
+    @test endswith(y)(y)
     @test endswith(z, z)
+    @test endswith(z)(z)
     x = SubString(x)
     y = SubString(y)
     z = SubString(z)
     @test !endswith(x, y)
+    @test !endswith(y)(x)
     @test !endswith(x, z)
+    @test !endswith(z)(x)
     @test endswith(y, z)
+    @test endswith(z)(y)
     @test endswith(x, x)
+    @test endswith(x)(x)
     @test endswith(y, y)
+    @test endswith(y)(y)
     @test endswith(z, z)
+    @test endswith(z)(z)
     x = "x∀y"
     y = SubString("x\x88\x80y", 2, 4)
     z = SubString("x\x80y", 2, 3)
     @test !endswith(x, y)
+    @test !endswith(y)(x)
     @test !endswith(x, z)
+    @test !endswith(z)(x)
     @test endswith(y, z)
+    @test endswith(z)(y)
     @test endswith(x, x)
+    @test endswith(x)(x)
     @test endswith(y, y)
+    @test endswith(y)(y)
     @test endswith(z, z)
+    @test endswith(z)(z)
 end
 
 @testset "filter specialization on String issue #32460" begin