98 lines
3.7 KiB
ReStructuredText
98 lines
3.7 KiB
ReStructuredText
.. _go-diagnostics:
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Diagnostic Messages
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===================
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An important concern for any machine language intended for human authoring is
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to produce good error messages when the input is somehow invalid, or has
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other problems.
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HCL uses *diagnostics* to describe problems in an end-user-oriented manner,
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such that the calling application can render helpful error or warning messages.
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The word "diagnostic" is a general term that covers both errors and warnings,
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where errors are problems that prevent complete processing while warnings are
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possible concerns that do not block processing.
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HCL deviates from usual Go API practice by returning its own ``hcl.Diagnostics``
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type, instead of Go's own ``error`` type. This allows functions to return
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warnings without accompanying errors while not violating the usual expectation
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that the absense of errors is indicated by a nil ``error``.
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In order to easily accumulate and return multiple diagnostics at once, the
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usual pattern for functions returning diagnostics is to gather them in a
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local variable and then return it at the end of the function, or possibly
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earlier if the function cannot continue due to the problems.
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.. code-block:: go
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func returningDiagnosticsExample() hcl.Diagnostics {
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var diags hcl.Diagnostics
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// ...
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// Call a function that may itself produce diagnostics.
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f, moreDiags := parser.LoadHCLFile("example.conf")
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// always append, in case warnings are present
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diags = append(diags, moreDiags...)
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if diags.HasErrors() {
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// If we can't safely continue in the presence of errors here, we
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// can optionally return early.
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return diags
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}
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// ...
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return diags
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}
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A common variant of the above pattern is calling another diagnostics-generating
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function in a loop, using ``continue`` to begin the next iteration when errors
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are detected, but still completing all iterations and returning the union of
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all of the problems encountered along the way.
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In :ref:`go-parsing`, we saw that the parser can generate diagnostics which
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are related to syntax problems within the loaded file. Further steps to decode
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content from the loaded file can also generate diagnostics related to *semantic*
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problems within the file, such as invalid expressions or type mismatches, and
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so a program using HCL will generally need to accumulate diagnostics across
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these various steps and then render them in the application UI somehow.
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Rendering Diagnostics in the UI
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-------------------------------
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The best way to render diagnostics to an end-user will depend a lot on the
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type of application: they might be printed into a terminal, written into a
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log for later review, or even shown in a GUI.
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HCL leaves the responsibility for rendering diagnostics to the calling
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application, but since rendering to a terminal is a common case for command-line
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tools, the `hcl` package contains a default implementation of this in the
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form of a "diagnostic text writer":
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.. code-block:: go
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wr := hcl.NewDiagnosticTextWriter(
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os.Stdout, // writer to send messages to
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parser.Files(), // the parser's file cache, for source snippets
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78, // wrapping width
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true, // generate colored/highlighted output
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)
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wr.WriteDiagnostics(diags)
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This default implementation of diagnostic rendering includes relevant lines
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of source code for context, like this:
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::
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Error: Unsupported block type
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on example.tf line 4, in resource "aws_instance" "example":
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2: provisionr "local-exec" {
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Blocks of type "provisionr" are not expected here. Did you mean "provisioner"?
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If the "color" flag is enabled, the severity will be additionally indicated by
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a text color and the relevant portion of the source code snippet will be
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underlined to draw further attention.
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