ch08

Introduction to Gofer                                         8. ERRORS


8. ERRORS

8.1  Errors detected on input
-----------------------------
After an expression has been entered, but before any attempt is made to
evaluate it, Gofer carries out a number of checks to make sure that the
expression that you typed does not contain any errors.  Here  are  some
examples of the kind of problem that might occur:

  o  Syntax errors.  The most common situation in which this happens is
     when  you  make  a  typing  mistake,  either  leaving   out   some
     characters, or perhaps pressing the wrong keys  instead.   In  the
     following example, the user has missed out a `[' character:

         ? sum 1..100]
         ERROR: Syntax error in input (unexpected `..')
         ?

  o  Undefined variables.  This happens when you  enter  an  expression
     using a variable or function name that is not defined  in  any  of
     the files of definitions loaded into Gofer.  This can  often  mean
     that you have misspelt the name of a function, or that  the  files
     defining a function have not yet been loaded.  For example:

         ? sum [1..n]
         ERROR: Undefined variable "n"
         ? 

  o  Type errors.  Certain  expressions  are  sensible  only  when  the
     functions used in those expressions are applied to values  of  the
     appropriate type.  For example, whilst the factorial function  can
     be used to calculate the factorial of an integer,  it  is  clearly
     meaningless to try to  determine  the  factorial  of  a  character
     value.  This kind of problem can be detected using  the  types  of
     the components of an expression.  In the expression "fact 'A'", we
     can see that the argument 'A' has type Char which does  not  match
     the argument type Int of the factorial function.  This error  will
     be detected by Gofer if you try to evaluate the expression:

         ? fact 'A'
         ERROR: Type error in application
         *** expression     : fact 'A'
         *** term           : 'A'
         *** type           : Char
         *** does not match : Int

         ?


8.2  Errors during evaluation
-----------------------------
If no errors are detected in an input expression, Gofer then begins  to
evaluate that expression.  Despite all of the checks that  are  carried
out before the evaluation begins, it is still possible for an error  to
occur during the evaluation of an expression.   A  typical  example  of
this is an attempt to divide a number by zero.   In  this  case,  Gofer


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Introduction to Gofer                     8.2  Errors during evaluation


prints the part of the  expression  being  evaluated  that  caused  the
error, surrounded by braces `{' and `}':

    ? 3/0
    {primDivInt 3 0}
    (4 reductions, 30 cells)
    ? 

[The function "primDivInt" which appears here is a  primitive  function
used to divide  one  integer  (its  first  argument)  by  another  (the
second)].  If an error occurs in just one part of  an  expression, only
the part causing the problem will be displayed:

    ? 4 + (5/0)
    {primDivInt 5 0}
    (5 reductions, 32 cells)
    ? 

A standard function called "error" is defined in the  standard  prelude
which is often useful for ensuring that appropriate error messages  are
produced when an error occurs:

    ? error "Problem has occurred"
    {error "Problem has occurred"}
    (23 reductions, 99 cells)
    ? 
































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