Add peek(io, T)

Author: ararslan

base/essentials.jl

@@ -787,9 +787,9 @@ ismissing(::Missing) = true
 function popfirst! end
 
 """
-    peek(stream)
+    peek(stream[, T=UInt8])
 
-Read and return the next value from a stream without advancing the current position
+Read and return a value of type `T` from a stream without advancing the current position
 in the stream.
 
 # Examples
@@ -802,7 +802,13 @@ julia> peek(b)
 
 julia> position(b)
 0
+
+julia> peek(b, Char)
+'j': ASCII/Unicode U+006A (category Ll: Letter, lowercase)
 ```
+
+!!! compat "Julia 1.5"
+    The method which accepts a type requires Julia 1.5 or later.
 """
 function peek end
 

base/io.jl

@@ -251,14 +251,16 @@ function unsafe_read(s::IO, p::Ptr{UInt8}, n::UInt)
     nothing
 end
 
-function peek(s::IO)
+function peek(s::IO, ::Type{T}) where T
     mark(s)
-    try read(s, UInt8)
+    try read(s, T)
     finally
         reset(s)
     end
 end
 
+peek(s) = peek(s, UInt8)
+
 # Generic `open` methods
 
 """

base/iobuffer.jl

@@ -172,7 +172,7 @@ function unsafe_read(from::GenericIOBuffer, p::Ptr{UInt8}, nb::UInt)
     nothing
 end
 
-function read(from::GenericIOBuffer, T::Union{Type{Int16},Type{UInt16},Type{Int32},Type{UInt32},Type{Int64},Type{UInt64},Type{Int128},Type{UInt128},Type{Float16},Type{Float32},Type{Float64}})
+function peek(from::GenericIOBuffer, T::Union{Type{Int16},Type{UInt16},Type{Int32},Type{UInt32},Type{Int64},Type{UInt64},Type{Int128},Type{UInt128},Type{Float16},Type{Float32},Type{Float64}})
     from.readable || _throw_not_readable()
     avail = bytesavailable(from)
     nb = sizeof(T)
@@ -183,7 +183,12 @@ function read(from::GenericIOBuffer, T::Union{Type{Int16},Type{UInt16},Type{Int3
         ptr::Ptr{T} = pointer(from.data, from.ptr)
         x = unsafe_load(ptr)
     end
-    from.ptr += nb
+    return x
+end
+
+function read(from::GenericIOBuffer, T::Union{Type{Int16},Type{UInt16},Type{Int32},Type{UInt32},Type{Int64},Type{UInt64},Type{Int128},Type{UInt128},Type{Float16},Type{Float32},Type{Float64}})
+    x = peek(from, T)
+    from.ptr += sizeof(T)
     return x
 end
 
@@ -216,7 +221,7 @@ end
     return byte
 end
 
-function peek(from::GenericIOBuffer)
+function peek(from::GenericIOBuffer, ::Type{UInt8})
     from.readable || _throw_not_readable()
     if from.ptr > from.size
         throw(EOFError())

base/secretbuffer.jl

@@ -154,7 +154,7 @@ bytesavailable(io::SecretBuffer) = io.size - io.ptr + 1
 position(io::SecretBuffer) = io.ptr-1
 eof(io::SecretBuffer) = io.ptr > io.size
 isempty(io::SecretBuffer) = io.size == 0
-function peek(io::SecretBuffer)
+function peek(io::SecretBuffer, ::Type{UInt8})
     eof(io) && throw(EOFError())
     return io.data[io.ptr]
 end

base/stream.jl

@@ -1193,9 +1193,9 @@ unmark(x::LibuvStream)   = unmark(x.buffer)
 reset(x::LibuvStream)    = reset(x.buffer)
 ismarked(x::LibuvStream) = ismarked(x.buffer)
 
-function peek(s::LibuvStream)
+function peek(s::LibuvStream, ::Type{T}) where T
     mark(s)
-    try read(s, UInt8)
+    try read(s, T)
     finally
         reset(s)
     end

stdlib/Markdown/src/Common/block.jl

@@ -19,7 +19,7 @@ function paragraph(stream::IO, md::MD)
     while !eof(stream)
         char = read(stream, Char)
         if char == '\n' || char == '\r'
-            char == '\r' && !eof(stream) && Char(peek(stream)) == '\n' && read(stream, Char)
+            char == '\r' && !eof(stream) && peek(stream, Char) == '\n' && read(stream, Char)
             if prev_char == '\\'
                 write(buffer, '\n')
             elseif blankline(stream) || parse(stream, md, breaking = true)

stdlib/Markdown/src/Julia/interp.jl

@@ -9,7 +9,7 @@ end
 
 function interpinner(stream::IO, greedy = false)
     startswith(stream, '$') || return
-    (eof(stream) || Char(peek(stream)) in whitespace) && return
+    (eof(stream) || peek(stream, Char) in whitespace) && return
     try
         return _parse(stream::IOBuffer, greedy = greedy)
     catch e

stdlib/Markdown/src/parse/parse.jl

@@ -53,9 +53,7 @@ function parseinline(stream::IO, md::MD, config::Config)
     content = []
     buffer = IOBuffer()
     while !eof(stream)
-        # FIXME: this is broken if we're looking for non-ASCII
-        # characters because peek only returns a single byte.
-        char = Char(peek(stream))
+        char = peek(stream, Char)
         if haskey(config.inner, char) &&
                 (inner = parseinline(stream, md, config.inner[char])) !== nothing
             c = String(take!(buffer))

stdlib/Markdown/src/parse/util.jl

@@ -14,7 +14,7 @@ const whitespace = " \t\r"
 Skip any leading whitespace. Returns io.
 """
 function skipwhitespace(io::IO; newlines = true)
-    while !eof(io) && (Char(peek(io)) in whitespace || (newlines && peek(io) == UInt8('\n')))
+    while !eof(io) && (peek(io, Char) in whitespace || (newlines && peek(io) == UInt8('\n')))
         read(io, Char)
     end
     return io
@@ -180,7 +180,7 @@ function parse_inline_wrapper(stream::IO, delimiter::AbstractString; rep = false
         startswith(stream, delimiter^n) || return nothing
         while startswith(stream, delimiter); n += 1; end
         !rep && n > nmin && return nothing
-        !eof(stream) && Char(peek(stream)) in whitespace && return nothing
+        !eof(stream) && peek(stream, Char) in whitespace && return nothing
 
         buffer = IOBuffer()
         while !eof(stream)

stdlib/REPL/src/REPL.jl

@@ -542,7 +542,7 @@ function hist_from_file(hp, file, path)
         while !isempty(line)
             push!(lines, chomp(line[2:end]))
             eof(file) && break
-            ch = Char(peek(file))
+            ch = peek(file, Char)
             ch == ' '  && error(munged_history_message(path), countlines)
             ch != '\t' && break
             line = hist_getline(file)

test/iobuffer.jl

@@ -332,3 +332,12 @@ end
     @test readline(b) == "Goodbye!"
     close(b)
 end
+
+@testset "peek(::GenericIOBuffer)" begin
+    io = Base.GenericIOBuffer(UInt8[], true, true, false, true, typemax(Int))
+    write(io, "こんにちは")
+    @test peek(io) == 0xe3
+    @test peek(io, Char) == 'こ'
+    @test peek(io, Int32) == -476872221
+    close(io)
+end

test/secretbuffer.jl

@@ -11,6 +11,8 @@ using Test
         @test read(secret, String) == str
         seekstart(secret)
 
+        @test peek(secret) == 0x66
+
         @test shred!(secret) === secret
         @test read(secret, String) == ""
         @test str == "foobar"