Question

EDIT: Based on answers below I added “dbo.” and comments for clarification.

Question: How do I pass a SQL user-defined function (UDF) as an argument for another UDF? Note that the function returns a table.

select dbo.fn_Scalar()                      -- Returns 'sa'.
select * from dbo.fn_Tabler('sa')           -- Returns a table (not scalar).
select dbo.fn_ScalarArg(dbo.fn_Scalar())    -- Returns 'sa'
select * from dbo.fn_Tabler(dbo.fn_Scalar())    -- ERROR: Incorrect syntax near '.'.

(Currently using SQL Server 2008 R2)

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EDIT: To ease debugging, here is the sample code to create the above functions:

IF EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[fn_Scalar]') AND type in (N'FN', N'IF', N'TF', N'FS', N'FT'))
DROP FUNCTION dbo.fn_Scalar
GO
IF EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[fn_Tabler]') AND type in (N'FN', N'IF', N'TF', N'FS', N'FT'))
DROP FUNCTION dbo.fn_Tabler
GO
IF EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[fn_ScalarArg]') AND type in (N'FN', N'IF', N'TF', N'FS', N'FT'))
DROP FUNCTION dbo.fn_ScalarArg
GO

create function fn_Scalar()
returns char(200) as
begin
    declare @user char(200)
    select  @user = 'sa'
    return  @user
end
GO

create function fn_Tabler(@User char(200))
returns table as
    return select @User [User]
GO

create function fn_ScalarArg(@User char(200))
returns char(200) as
begin
    declare @u2 char(200)
    select  @u2 = @User
    return  @u2
end
go

Answer 1

BOL says: “Scalar-valued functions must be invoked by using at least the two-part name of the function.”

select * from dbo.fn_Tabler(dbo.fn_Scalar())    

EDIT:
I was wrong, there is the table-valued function… Documentation says that only constants and @local_variables can be passed to table-valued functions (see here)