More Ada Constructs 7 Oct. 2002

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More AdaConstructs7 Oct. 2002
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Todays Constructs

• Derived types
• Record types
• Array types
• Visibility and scope
• Subprogram arguments
• Exception handling
• Case statement

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Derived Types

• Ada is a strongly-typed language
• No automatic conversions
• Different types can only interact using
  operations specifically defined for the types
• To even add more strength to types, Ada allows
  the user to create new types derived from other
  types.
• type Meters_t is new Float
• type Yards_t is new Float
• Distance_To_Paris Meters_t
• Distance_To_NY Yards_t
• Distance_To_Paris Distance_To_NY 1000.0 --
  Illegal!
• However, explicit conversions can be performed
• Distance_To_Paris Meters_t (Distance_To_NY
  0.9144) 1000.0

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Record Types

• Comparable to structures in other languages
• One object contains components of various types
• type Vehicle_State is record
• Position Position_type
• Velocity Velocity_type
• Linear_Acceleration Linear_Acceleration_type
• Inertial_Attitude Attitude_type
• Attitude_Rate Attitude_Rate_type
• Att_Accel Att_Accel_type -- Yes, you can
  abbreviate
• end record
• Ada records also can have discriminants

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Discriminated Records

• type Matrix_t is array (Int_32_t range ltgt,
  Int_32_t range ltgt) of Single_Float_t
• Want to have a Trace function for a square matrix
• Would need to check for non-square
• Would need to check ranges (someone could have
  created an object My_Array Matrix_t (1 .. 3, 0
  .. 2) )
• Instead, create a discriminated record type
• subtype Pos_32_t is Int_32_t range 1 ..
  Int_32_tlast
• type Square_Matrix_t (Order Pos_32_t 3) is
  record
• Matrix Matrix_t (1 .. Order, 1 .. Order)
• end record
• Now, the compiler will make sure the input is of
  the correct form

Taken from Programming in Ada95, 2nd Edition,
John Barnes
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Variant Records

• The discriminant can also be used to create
  different records depending on the value of the
  discriminant
• type Error_t is (None, RAM_Scrub, Exception)
• type One_Hz_Telem_Data_t (Error_Kind Error_t
  None) is record
• case Error_Kind is
• when None gt
• null
• when RAM_Scrub gt
• Bad_Location Address_t
• when Exception gt
• Exception_data Exception_Info_t
• end case
• end record

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Discriminant Can Change Dynamically or Can be
Constant

• M Square_Matrix_t (3, (1 .. 3 gt (1 .. 3 gt
  0.0)))
• N Square_Matrix_t (Order gt 4) (4, (1 .. 4
  gt (1 .. 4 gt 1.0)))
• .
• .
• .
• M N -- Legal
• N M -- Illegal (Order can not change for N)

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Array Types

• Array types are easy
• Multidimensional arrays are allowed
• Matrix_t seen previously
• Arrays of any other types are allowed, including
  arrays
• Different syntax
• A (1,2) for 2-dim array
• A (1) (2) for array of arrays
• Very good idea to always have a well-constrained
  subtype for array types (or discriminated records
  with arrays)
• M Square_Matrix_t -- How much memory will
  this take?

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Visibility and Scope

• Good News Not every name used in multiple files
  must be global
• In fact, only Library-level names are globally
  defined
• Library-level names are packages and subprograms
  that are declared at the outermost level of a
  compilation unit
• Almost all global names are packages
• One notable exception function
  Unchecked_Conversion
• Bad News Lots of typing to tell the compiler
  what you want
• Ada.Text_IO.Integer_IO.Put
• Not-so-bad News use statement renames
  statement
• with Ada.Text_IO
• ...
• use Ada.Text_IO.Integer_IO -- Makes all
  declarations in package
• -- locally visible

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Scope Example

1. procedure Level_1 is
2. Count, Index INTEGER 1
3. procedure Level_2 is
4. Index, Count INTEGER 2
5. procedure Level_3 is
6. Count INTEGER 3
7. begin
8. Count
   -- Count from line 6
9. Level_1.Count
   -- Count from line 2
10. end Level_3
11. procedure Level_3_Prime is
12. Data, Index, Count INTEGER 3
13. Outer_Index INTEGER renames
    Level_1.Level_2.Index
14. begin
15. Count Outer_Index
    -- Count from line 12
16. Level_1.Level_2.Count
    -- Count from line 4
17. end Level_3_Prime
18. begin
19. null

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The Use Clause

• Makes names visible without qualification
• Extremely useful to make readable code
• Biggest problem is enumeration literals
• package Drawing_Package is
• type Colors_t is (Red, Blue, Yellow)
• . . .
• with Drawing_Package
• procedure Use_It is
• C Drawing_Package.Colors_t
• Drawing_Package.Red -- Tedious
• Somewhat bad in that it makes it more difficult
  to find things
• Majority opinion Use sparingly, so maintainers
  can easily find things
• Minority opinion Tools can help find things
  Ada has enough necessary tedium dont impose more

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Subprogram Arguments

• Procedures can have arguments of 3 kinds
• in means treat as a constant inside procedure
• out means it must be written before it is read
  (similar to an uninitialized variable)
• The value is given to the caller when the
  procedure exits
• in out can be read and written
• The value is copied in at the start, and copied
  out at the end
• Note that copying or referencing is meant for
  example the actual parameter passing mechanism
  is defined differently for different kinds of
  data, and in some cases is left up to the compiler

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Exception Handling

• Exceptions are handled at the innermost scope
  where there is a relevant exception handler, out
  to the task level
• For the moment, assume only 1 task in the program
• X F(G(H(I(J(X)))))
• If an exception occurs in function J, if J has an
  exception handler for the specific exception, it
  will be handled locally
• None of the outer functions will know the
  exception occurred
• If J does not have an exception handler for that
  exception, it is propagated to the next level

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Exception Propagation

• with Ada.Text_IO use Ada.Text_IO
• procedure Exception_Test is
• type My_Int is range 1 .. 5
• Y My_Int
• function Level_1 (X My_Int) return My_Int is
• Y My_Int
• begin
• Y X 10 -- raises Constraint_Error
• . . . -- Does not matter. Never executes
  these statements
• return X -- Does not execute this
• exception
• when Program_Error gt
• return My_IntLast -- Attributes are useful
• when End_Error gt
• return My_IntFirst
• end Level_1
• begin
• Y Level_1 (1)
• put (Made it)

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Output

• Constraint_Error in Y
• Program exited normally

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What Happens if no Exception Handler?

• raised CONSTRAINT_ERROR exception_test.adb8
• Exception propagated to outermost level of
  current task
• Task (if no exception handler) ends
• Up to runtime system what to do next
• For GNAT, see above for single task case
• For multi-task case, tasks just disappear
• GVD allows breakpoint on any exception
• AdaGIDE has option to print traceback
• Shows the call stack when run inside AdaGIDE

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Case Statement

• X My_Positive 1
• . . .
• case (X) is
• when 1 gt
• . . . -- Any sequence of statements (if X 1)
• when 2 gt
• . . . -- Any sequence of statements (if X 2)
• when 3 .. 10 gt
• . . . -- Any sequence of statements (if X 3,
  4, or 5)
• when others gt
• . . . -- Any sequence of statements (X
  anything else)
• end case
• Note, no need for break statement after
  relevant sequence of statements, jump to end
  case