Accept (i, name) -> Bool functions for select and drop

Project.toml

@@ -1,6 +1,6 @@
 name = "CSV"
 uuid = "336ed68f-0bac-5ca0-87d4-7b16caf5d00b"
-version = "1.1.0"
+version = "1.2.0"
 authors = ["Jacob Quinn <quinn.jacobd@gmail.com>"]
 
 [deps]

docs/src/examples.md

@@ -391,17 +391,19 @@ file = CSV.File(IOBuffer(data); select=[:a, :c])
 file = CSV.File(IOBuffer(data); select=["a", "c"])
 file = CSV.File(IOBuffer(data); select=[true, false, true])
 file = CSV.File(IOBuffer(data); select=r"[ac]")
+file = CSV.File(IOBuffer(data); select=(i, name) -> name in (:a, :c))
 # now examples of dropping, when we'd rather specify the column(s)
 # we'd like to drop/exclude from parsing
 file = CSV.File(IOBuffer(data); drop=[2])
 file = CSV.File(IOBuffer(data); drop=[:b])
 file = CSV.File(IOBuffer(data); drop=["b"])
 file = CSV.File(IOBuffer(data); drop=[false, true, false])
 file = CSV.File(IOBuffer(data); drop=:b)
+file = CSV.File(IOBuffer(data); drop=(i, name) -> i == 2)
 ```
 
-`select` and `drop` take a list, one name or index,
-or a `Regex`; the selected columns keep their file order.
+`select` and `drop` take a list, one name or index, a `Regex`, or an
+`(i, name) -> Bool` function; the selected columns keep their file order.
 
 ## [Limiting number of rows from data](@id limit_example)
 

docs/src/migration.md

@@ -57,11 +57,12 @@ throws), and extra trailing zeros are exact.
 The `PosLenString` output type is retired. Use `DataStrings.DataString`, `String`,
 or an InlineStrings.jl type.
 
-`types`, `select`, `drop`, and `pool` no longer accept functions. Use type or
-policy values, vectors, or dictionaries. `select` and `drop` take a list, one
-name or index, or a `Regex`; they work on `CSV.File`, `CSV.lazy`, `CSV.Rows`,
-and `CSV.Chunks` and return a unique file-ordered column set. Use
-`Tables.Scan` for a serializable projection and filter expression.
+`types` and `pool` no longer accept functions. Use type or policy values,
+vectors, or dictionaries. `select` and `drop` take a list, one name or index, a
+`Regex`, or the 0.10 `(i, name) -> Bool` function; they work on `CSV.File`,
+`CSV.lazy`, `CSV.Rows`, and `CSV.Chunks` and return a unique file-ordered
+column set. Use `Tables.Scan` for a serializable projection and filter
+expression.
 
 ## Table access
 

docs/src/reading.md

@@ -223,8 +223,11 @@ numbers infer as `Float64`.
 name or index, or a `Regex` matched against the column names. They are
 mutually exclusive. `CSV.File`, `CSV.lazy`, `CSV.Rows`, and `CSV.Chunks` all
 return selected columns once, in file order, even when the list is repeated or
-reordered. Function-valued selection is not supported. Use a `Tables.Scan`
-request for expression-based projection and filtering.
+reordered. A function of the form `(i, name) -> Bool` receives each column's
+position and name (a `Symbol`); `select` keeps and `drop` removes the columns
+for which it returns `true`. It sees only header names, so it runs before any
+value is read, and CSV can call it more than once for a column. Use a
+`Tables.Scan` request for expression-based projection and filtering.
 
 CSV.jl uses `DataStrings.DataString` for inferred text columns by default:
 

docs/src/release-notes.md

@@ -1,3 +1,20 @@
+# CSV.jl 1.2.0 release notes
+
+Released October 15, 2026. Requires Julia 1.10 or later.
+
+## Changes
+
+- `select` and `drop` accept an `(i, name) -> Bool` function again, as in 0.10.
+  It receives each column's position and name before any value is read. 1.0
+  and 1.1 rejected it. `types` and `pool` still do not accept functions.
+
+## Fixes
+
+- Delimiter detection only elects a delimiter that splits the first row. A
+  single-column file of times or timestamps, such as `event time` over
+  `10:30:00` rows, is read as one column again instead of being split on `:`,
+  as in 0.10.
+
 # CSV.jl 1.1.0 release notes
 
 Released September 21, 2026. Requires Julia 1.10 or later.

src/CSV.jl

@@ -79,7 +79,8 @@ become `missing` by default; `strict=true` or `on_error=:error` throws a
 [`CSV.ParseError`](@ref CSV.ParseError) when the cell is accessed. Rows do not
 retain parse diagnostics, so use [`CSV.File`](@ref CSV.File) when
 `CSV.problems` or a diagnostic cap is needed. `select` and `drop` (a list, one
-name, or a `Regex`) project columns in stable file order.
+name, a `Regex`, or an `(i, name) -> Bool` function) project columns in stable
+file order.
 """ Rows
 @doc """
     CSV.Chunks(source; ntasks=Threads.nthreads(), chunkbytes=nothing, keywords...)

src/api.jl

@@ -18,8 +18,9 @@
 #   • problems are retained data; eager readers also warn once by default
 #     (`strict=true` maps to `on_error=:error`, `maxwarnings` to `maxproblems`)
 #   • empty unquoted cells are ALWAYS missing; `missingstring` ADDS spellings
-#   • `select`/`drop`/`types` take lists, names, regexes, or dictionaries
-#     (Tables.Scan is the expression channel)
+#   • `select`/`drop`/`types` take lists, names, regexes, or dictionaries;
+#     `select`/`drop` also take a `(i, name) -> Bool` function over header
+#     names (Tables.Scan is the expression channel)
 #   • `stringtype` defaults to DataString; `stringtype=String` materializes;
 #     InlineStrings are an extension
 #   • Bool defaults accept lower, title, and upper case; user lists replace them

src/core.jl

@@ -3480,9 +3480,12 @@ function _selectpositions(select, drop, names::Vector{Symbol};
         throw(ArgumentError("select and drop are mutually exclusive"))
     spec = select === nothing ? drop : select
     spec === nothing && return collect(eachindex(names))
-    spec isa Base.Callable &&
-        throw(ArgumentError("function-typed select/drop is retired; pass a list, " *
-                            "a Regex, or use Tables.Scan for expressions"))
+    if spec isa Base.Callable
+        # a `(i, name) -> Bool` function sees header names only, so it becomes
+        # the Bool mask before any value is read
+        f = spec   # the comprehension captures a single-assignment name
+        spec = Bool[f(i, nm)::Bool for (i, nm) in enumerate(names)]
+    end
     if spec isa Regex
         re = spec   # the comprehension captures a single-assignment name
         matched = [nm for nm in names if occursin(re, String(nm))]

test/api.jl

@@ -9,7 +9,6 @@
 #   • empty unquoted cells are ALWAYS missing (custom missingstring ADDS)
 #   • long rows do not widen the schema (extra fields ⇒ problem, not Column4)
 #   • diagnostics are retained data, with one summary warning by default
-#   • function-typed select/drop retired
 #   • wide integers that fit Int128 remain exact
 
 using Test, Dates, Tables, PooledArrays, CodecZlib, InlineStrings, FilePathsBase, Random, Mmap, Parsers
@@ -496,7 +495,7 @@ end
     sourceparity(input; kw=(; drop=[2]))
     sourceparity(input; kw=(; drop=[false, true, false]))
     @test_throws ArgumentError A.File(IOBuffer(input); select=[:a], drop=[:b])
-    @test_throws ArgumentError A.File(IOBuffer(input); select=(nm, i) -> i == 1)
+    @test_throws ArgumentError A.File(IOBuffer(input); select=(i, nm) -> i == 1, drop=[:b])
     @test_throws ArgumentError A.File(IOBuffer(input); select=[:nope])
     f = sourceparity("my col,b\n1,2\n"; kw=(; normalizenames=true, select=[:my_col]))
     @test Base.names(f) == [:my_col]
@@ -2428,7 +2427,28 @@ end
     @test names(first(A.Chunks(IOBuffer(src); drop=r"^a"))) == [:b]
     @test_throws ArgumentError A.File(IOBuffer(src); select=r"^z")
     @test names(A.File(IOBuffer(src); drop=r"^z")) == [:ax, :ay, :b]
-    @test_throws ArgumentError A.File(IOBuffer(src); select=(i, nm) -> true)
+end
+
+@testset "select and drop functions" begin
+    input = "a,b,c\n1,2,3\n4,5,6\n"
+    seen = Tuple{Int, Symbol}[]
+    f = sourceparity(input; kw=(; select=(i, nm) -> (push!(seen, (i, nm)); nm !== :b)))
+    @test Base.names(f) == [:a, :c] && f.c == [3, 6]
+    @test unique(seen) == [(1, :a), (2, :b), (3, :c)]
+    @test Base.names(sourceparity(input; kw=(; drop=(i, nm) -> i == 2))) == [:a, :c]
+    @test isempty(Base.names(A.File(IOBuffer(input); select=(i, nm) -> false)))
+    @test Base.names(A.File(IOBuffer(input); drop=(i, nm) -> false)) == [:a, :b, :c]
+    @test names(A.lazy(IOBuffer(input); drop=(i, nm) -> nm === :a)) == [:b, :c]
+    @test Tables.columnnames(A.Rows(IOBuffer(input); select=(i, nm) -> i > 1)) == [:b, :c]
+    @test all(ch -> names(ch) == [:a], A.Chunks(IOBuffer(input); select=(i, nm) -> i == 1))
+    @test names(A.File([IOBuffer(input), IOBuffer(input)]; drop=(i, nm) -> i == 1)) == [:b, :c]
+    # the function sees the names after normalizenames
+    @test Base.names(A.File(IOBuffer("my col,b\n1,2\n"); normalizenames=true,
+                            select=(i, nm) -> nm === :my_col)) == [:my_col]
+    # CSV.File(lazyfile) passes positions among the lazy file's columns
+    lf = A.lazy(IOBuffer(input); drop=[:a])
+    @test Base.names(A.File(lf; select=(i, nm) -> i == 1)) == [:b]
+    @test_throws TypeError A.File(IOBuffer(input); select=(i, nm) -> missing)
 end
 
 @testset "types=String names the output type" begin